Tag Archives: yellow

Colour Mixing

I really like this page by John Lovett about colour mixing.

We all know that you can’t mix all colours by starting from three primaries. You can’t do this in theory and you can’t do it in practice. You can’t do it with additive colour mixing and you can’t do it with subtractive mixing. In fact, with subtractive mixing, the oft-cited primaries of red, yellow and blue are actually not a very good choice.

Mixing red and blue pigments, for example, won’t give you a great purple. You will lose saturation and you almost certainly won’t get the vivid purple that is suggested by many colour wheels. However, John Lovett’s page explains how, if you do start with red, yellow and blue, you can do a little better by understanding that there is not just one blue and one red, for example. If you want to mix yellow and blue you should use a greenish yellow and a grreenish blue. On the other hand, if you want to mix blue and red you should use a reddish blue and a bluish red. This reduces the loss in saturation.

However, although Lovett’s advice is superb, you still can’t make all of the colours this way (though you can make all the hues of course). And arguably what Lovett is proposing is a six-primary system rather than a three-primary system. Lovett ends up proposing a six-primary system in an attempt to make the out-dated idea of RYB work.

Why yellow and blue don’t make green

[and why we should stop teaching it in schools]

You will find images like the one above, that show that red, yellow and blue are the primaries and that yellow and blue make green.

Sometimes this is represented as a colour wheel:

So some people say yellow and blue make green. And you will find other answers that say that yellow and blue make black. How can this be?

Well, we need to understand a little science to get to the bottom of this.

The figure below shows what happens when you mix an ideal yellow dye with an ideal blue dye. The blue dye reflects light perfectly in about a third of the spectrum (and absorbs perfectly in the other two thirds). The yellow pigment reflects light perfectly in about two thirds of the spectrum (and absorbs perfectly in the other third).

The problem here is that the blue and yellow pigments (between them) absorb perfectly across the whole spectrum. The people who say that yellow and blue make black are saying so because of this argument.

Note that blue is a particularly bad choice of primary because it absorbs so broadly across the spectrum. [Making the blue even purer would only make the problem worse by the way.] Yellow is a good choice of subtractive primary because it only absorbs in one third of the spectrum.

The problem is, the people who say that blue and yellow make black are wrong of course. Every child knows this. In practice, if we measure the reflectance spectra for blue and yellow pigments they don’t look like those ideal ones I showed above. For a start, they are quite smooth. Here is a reflectance spectrum for a real yellow pigment. (The reflectance factor, by the way, is the proportion – or per cent – of light that the colorant reflects at each wavelength.)

Notice that with a real yellow colorant, it does not reflect perfectly in the middle and long wavelengths and it does not absorb perfectly in the short wavelengths. It reflects and absorbs to some extent all the wavelengths but it absorbs more at the shorter wavelength and absorbs at less the middle and longer wavelengths. The same is true of a real blue colorant; it does not absorb perfectly at the middle and longer wavelengths. The consequence of this is that you don’t get black if you mix blue and yellow. You would get black if the pigments were ideal but they are not. We live in the real world. However, you certainly don’t get a lovely bright green as shown in the colour wheel with red, yellow and blue primaries. You would get a dark desaturated murky dirty greenish colour. The main reason for this is that the blue is absorbing too broadly. Interestingly, if you look at the artist John Lovett’s page he explains that to mix a yellow and blue you should use a yellowish blue (and a bluish yellow). 

Now let’s see what happens when we mix cyan and yellow dyes. We’ll start with the ideal colours.

It’s very nice. We get a lovely green colour. Cyan is a great subtractive primary because unlike blue it absorbs in only one third of the spectrum (the red or long wavelengths). Note that it is precisely because the cyan does not look pure that makes it a great primary – that’s why I get so furious about people saying the primaries are pure colours. The cyan looks bluish-green because it reflects in two thirds of the spectrum and only absorbs in the reddish part. Neither the cyan nor the yellow dye absorb in the middle (green) part of the spectrum and therefore the result of mixing cyan and yellow is a lovely green. Except it is not quite true. Remember, this is for ideal pigments. Real dyes do not look like that. Refer back to the measured reflectance spectrum for the real yellow pigment. In reality cyan and yellow do make green but the green might be a little less saturated than you may wish for because of the unwanted absorptions by the two dyes in the areas of the spectrum where ideally they would not absorb. (It was the great Robert Hunt, who worked for many years at Kodak – for those who knew him – who taught me about unwanted absorptions.)

Have you ever seen this happen. Of course, you have. Whenever you use a printer (which typically uses cyan, magenta and yellow primaries) to get a green, the printer is using cyan and yellow to make the green.

Remember those people who say that you can’t make blue because – yawn – it’s a pure colour that can’t be made by mixture? Well, have you ever printed out blue on a printer? Of course, you have. Let’s look again at our ideal primaries and see if we can explain it.

That’s right. Mixing cyan and magenta makes blue. The cyan absorbs in one third (the red third) and the magenta absorbs in one third (the green third) but neither absorb the short wavelengths.

John Lovett explains that you can do a decent job of mixing red, yellow and blue dyes, but only if you allow yourself to use multiple blues and multiple yellows, for example. If you want to do the best job possible using only three subtractive primaries, then the best you can do is to use cyan, magenta and yellow. 

So finally you can see that the best subtractive primaries are cyan, magenta and yellow because the cyan is red absorbing, the magenta is green absorbing and the yellow is blue absorbing. And what is more, you now understand why this is the case (rather than accepting dogma). You also understand why there is a relationship between the CMY of subtractive mixing and the RGB of additive mixing.

The optimal additive primaries are red, green and blue (I will cover this elsewhere). And for this reason the optimal subtractive primaries are cyan (red absorbing), magenta (green absorbing) and yellow (blue absorbing). 

But don’t be fooled by this lovely subtractive colour mixing diagram. You might not get such lovely blue, green and red colours when you mix real CMY primaries (either on your printer or with inks/paints). Why not? Because of the unwanted absorptions.

If you want to to know more you could do worse that get a copy of Measuring Colour, now in it’s 4th edition, and authored by Hunt and Pointer. 

This post gets quite a few hits so I will take this opportunity to direct you to my short series of youtube clips that describe the issues discussed in this post in a visual way. You can see them here. If you want something a bit more technical check out this short lecture on colour primaries or visit my patreon.

Or visit my Patreon page here for more analysis like this

changing button colour increased conversion

button

It seems that only recently companies are carrying out what is known as split testing or A/B testing. Put two designs of a web site out and see which does best. Recently one company did just that. They had one web site with a green call-to-action button (as shown above) and another with a yellow call-to-action button. Changing the call-to-action button from green to yellow resulted in a 187.4% increase in conversions to their website. Is there some effect that yellow light could have compared to green? For example, could yellow light make users more impulsive?

According to Erika Dickstein it may be nothing to do with yellow at all but simply to do with the contrast – the yellow stands out better and therefore is more noticeable. Certainly more research is needed in this area.

Has technology for Harry Potter’s Daily Prophet just arrived?

I believe that print as we know it is dead. I know that there are some arguing that print is having a resurgence – just as there are those who think that vinyl is on the way back for music – but reports that physical books are gaining ground at the expense of digital are just plain wrong as is explained in this article. I saw this before with digital images where people argued that digital images would never replace traditional photography because of quality and price. Well, of course, we know that the quality of digital images increased and the cost of getting them decreased (when I was a student in the 80s it would have been bizarre to imagine that everyone would have a couple of cameras on them at all times) – but it was not this that killed traditional photography and eventually put the giant Kodak out of business. What killed traditional photography was when you could go to a gig, take a photo, and share it almost instantly with your friends around the world. Traditional photography could never compete with this.

Some people prefer reading print to looking at a screen though I am not one of them. But imagine when an e-document feels like paper, is light and flexible, but you can carry a whole newspaper with you (not to mention all the novels you have ever read) by carrying just one piece of it. And it looks just like print.

E ink, the company behind the pigment-based, low-energy monochromatic displays found in many of today’s popular readers has worked out how to create up to 32,000 colours using almost the same technology. For the first time they can create colours at each pixel using yellow, cyan, magenta and white pigments. The new display is 20-inch with 2500 x 1600 resolution. The image below is rendered in this way. This leads to the possibility of having coloured moving images made out of ink – just like the Daily Prophet in the Harry Potter movies. Well, not quite like that yet. But it’s coming. More details here.

ink

What colour is your office?

Decorating-GOMIX-new-office-2

I just saw an interesting article by Kim Lachance Shandrow about how the colour of your office can affect productivity. The article refers to a paper (2007) in Color Research and Application (CRA) by Nancy Kwallek entitled Work week productivity, visual complexity, and individual environmental sensitivity in three offices of different color interiors. The paper suggests that the influences of interior colours on worker productivity were dependent upon individuals’ stimulus screening ability and time of exposure to the interior colours. CRA is a top quality academic journal that is peer reviewed and so I am respectful of the findings.

However, in Kim’s online article there is a lot of stuff that I am highly sceptical about. For example, she writes that “Red … increases the heart rate and blood flow upon sight.” Is this true? Is there really any evidence for this. I have two PhD students working in this area right now and I am far from sure that colour does affect heart rate and, if it does, the effects are probably tiny. And yet we can read statements like this all over the internet as if it is a fact beyond doubt. Other things she says that I take with a pinch of salt is that “green does not cause eye fatigue” and that “yellow triggers innovation.” Don’t get me wrong – I am very interested in how colour can be used to affect us emotionally, psychologically and behaviourally; it’s just there is a danger that if some things are said often enough (such as red increases your blood pressure or heart rate) then people start believing them even though there may be little evidence.

That said, you might find the infographic fun and it is well done. See the original and full article here.

eyes change colour?

reindeer

I didn’t realise how sophisticated reindeers are. It turns out they have two layers of fur to help them keep warm, are able to shrink the pads on their hooves to give then better grip, and can detect ultraviolet light which enables them too see in very dim light. And it also turns out that their eyes can change colour in winter so that their vision is more sensitive. Reindeers, like cats, have a reflective layer behind the retina (which is the inside of the eye ball where all the light-sensitive cells are) that helps them to see in dim light. This is why, if you see a cat at night, you might see the eyes shining; you are seeing light being reflected back at you from the cat’s tapetum lucidum (which is the technical term for the layer behind the retina). The light that shines back in most animals with this layer is golden but in reindeer it apparently shifts to blue in the winter. The shift to blue allows more light to be scattered and improves the vision of the animal.

The full paper can be read in the Proceedings of the Royal Society.

Where is colour mixing?

Imagine that we have three projection lamps at the back of a hall – one has a red filter and so produces a beam of red light, and the other two use filters to produce green and blue beams. We project these onto a white screen and get three circles of light (one, red, one green and one blue). We then move the angles of the projectors so that the circles of light overlap. We get something that looks rather like this:

ColourMixing

Where the red and green light overlap we get yellow. We get magenta and cyan for the other two binary mixtures. So,

red + green = yellow

red + blue = magenta

green + blue = cyan

This is called additive colour mixing as I am sure you know. And if we mix all three primaries we can achieve white (or other neutral colours). The primaries could be single wavelengths of light – so we could use a primary at, say, 700 nm (for the red) and one at 450 nm (blue) and one at 530 nm (green). So green light (530 nm) and red light (700 nm) additively mix together and generate yellow. When this happens what is being mixed and where does this mixing take place? Take a few moments to consider this before reading on.

Notice I said that they additively mix to generate yellow – I specifically avoided saying that they mix to generate yellow light. When I sat down with a couple of students last week and asked then what they though they said that the red and green light mixed together to create yellow light and when I pressed them, they went further to say that the yellow light was at about 575 nm.

visible-a

If we measure the part of the screen that is yellow we would see that we have some light at 700 nm and some at 530 nm. The wavelengths are not mixed; they don’t mix together to generate some third wavelength of light such as 575 nm. So no physical mixing takes place other than – I suppose one could argue – that the red and green lights are mixed in the sense that they are spatially coincident. But that’s not really mixing, for me, and certainly doesn’t even begin to explain why we have the sensation of yellow when we look at these wavelengths together. It also makes me think that additive colour mixing, if it can be said to occur anywhere in particular, occurs in the eye. And I do mean eye, not brain.

why I don’t like the colour wheel

There are many reasons why I don’t like colour wheels of the type shown below:

The first reason is because it perpetuates the myth that the subtractive primaries are red, yellow and blue whereas the fact is that red, yellow and blue produces a rather small gamut of colours. It is certainly not the best choice of subtractive primaries though it is taught as dogma in many art and design schools and throughout children’s education. The problem is that whenever two colours are mixed together there is saturation loss; that is, the resultant mixture ends up being more desaturated than the two components were. The saturation loss is greatest when mixing colours on the opposite side of the colour circle where the resultant mixture can be almost grey. However, for certain choices of primaries, the saturation loss is greater than for others. If red, yellow and blue are used as the primaries then of course it is possible to generate any other hue. However, there is significant saturation loss and the above colour wheel gives completely the wrong impression. It suggests that mixing blue and yellow together, for example, results in a really bright vivid green.

The reality of pigment mixing is much more like the triangular colour wheel shown below:

In the above diagram it can be seen that mixing together yellow and blue results in a really muddy dark green. The purple resulting from mixing blue and red is almost black!! Now it is possible to mix together a blue and a yellow to get a better green. For example, mixing a greenish blue with a yellow will give a much more vivid green. Mixing a bluish red with a greenish blue will result in a lovely purple. We have a name for a greenish blue and a blueish red – we call them cyan and magenta. A much better colour gamut is obtained if we start with the primaries, cyan, magenta and yellow.

Footnote: Some people may look at the triangular colour wheel and think that the reason the colours are dull is that the red, yellow, and blue primaries used are not ‘pure’ enough. Nothing could be farther from the truth. If it was possible to make really vivid and bright red and blue pigments then the resultant colour gamut would be even smaller. Fundamentally, red, yellow and blue just don’t make good subtractive primaries.