Tag Archives: additive colour mixing

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:


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.


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.

subtractive mixing – why not RGB?

In a previous post I spoke about the difference between additive and subtractive mixing and why the additive primaries are red, green and blue or RGB for short – http://colourware.wordpress.com/2009/07/13/additive-colour-mixing/

The chromaticity diagram – see http://colourware.wordpress.com/2009/09/28/colourchat-audiovisual-guide-to-the-chromaticity-diagram/ – has a very useful property. If you plot the chromaticities of two lights, then the straight line that joins the two points on the chromaticity diagram show you the additive mixtures that can be obtained by mixing together the two lights. If we take three lights, then the additive mixtures that can be obtained are defined by the triangle that is formed if the chromaticities are the vertices of the triangle. Ok – that’s a bit of a mouthful so let’s have a practical example. The triangle in the diagram below shows the gamut that can be achieved when we have three additive primaries that are positioned at the corners of the triangle.


 From this diagram it should become obvious why the additive primaries are RGB. Say, we chose, two reds and a cyan as the three additive primaries – well, the triangle would be tiny. In other words, the gamut would not be very big. The biggest triangle in the chromaticity is one whose vertices are formed by a red, a green and a blue. WhichRGB will give the biggest triangle? I don’t know – it’s been something that has been puzzling me for the last few days and I’ll come back to this in a later post. But certainly any RGB triangle is pretty large as long as the red, green and blue primaries chosen are reasonably saturated.

So what happens if we choose RGB as the subtractive primaries? Subtractive colour mixing describes how inks and paints mix together to form colours. The first thing to point out is that subtractive colour mixing is not additive and linear – you remember I said that when you mix two lights together the colour mixtures all fall on the straight line that joins the  two points in the chromaticity diagram that represent the two lights? Well, this is only true for additive colour mixing. So to work out the gamut for subtractive systems is not an easy thing to do. However, if you do select the three subtractive primaries as RGB you’ll get a gamut that looks something like this:


Notice that the gamut is concave. Mixing red and green lights produces a nice yellow. You can test this by going into your colour-picker in software such as Photoshop or Powerpoint and setting the RGB values to be 255:255:0. You’ll get a nice yellow. But mixing red and green paints – it will give you a similar hue to yellow but you’ll get something quite desaurated; most likely you’ll get a brown. So using RGB as the subtractive primaries would not be a very good thing at all.

It turns out that additive and subtractive colour mixing are very related. The best subtractive primaries are the ones that control the amount of red, green and blue light reflected. A yellow dye applied to textiles, for example, mainly absorbs short wavelengths in the blue section of the spectrum, allowing the other wavelengths to be reflected by the textile. The “other wavelengths” that are reflected give yellow. But the important point is that the yellow dye absorbs blue. Similarly, a magenta dye absorbs green and a cyan dye absorbs red. This leads to the idea of the optimal subtractive primaries being those that are cyan, magenta and yellow or CMY. This leads to a gamut somewhat like this:


The biggest gamut for subtractive mixing is obtained by using CMY as the primaries. But weren’t you taught at school that the subtractive primaries are red, blue and yellow? Almost certainly you were – and this is because it is accepted dogma at most art colleges and in many art and design textbooks. But it is quite easy to show that the optimal primaries – those giving the largest gamut – are CMY not RBY. If you were building a colour-reproduction system using only three colours such as a printer you would come to the conclusion – as companies such as HP, Xerox, and Epson have done – that you get the largest colour range with CMY. So why has it become commonplace for artists to refer to red, yellow and blue as the primaries? Could it be a colour naming and language issue – that they really mean cyan when they say blue and it’s just a naming error. Possible, but not likely in my opinion.  I think it is more likely that most artists are not overly concerned that RYB gives a smaller gamut than CMY because they rarely restrict themselves to three primaries. An artist would typically use 6 or more primaries. For example, they might use two blues (one that is reddish and one that is greenish), two reds (one that is yellowish and one that is bluish) and two yellows (one that is greenish and one that is reddish) in order to easily be able to mix a wide range of colours. The (mis-)identification of RYB as the subtractive primaries has much to do with colour wheels. I like to keep each of these blog posts reasonably concise – if I start writing about the problems of colour wheels now I will be writing for another 2 hours. And it’s nearly midnight now so colour wheels will need to wait for another day!