Colour 101

I am really looking forward to some interesting topics such as

Is black a colour?


Does colour exist?

But, before I get into these tough topics I would like to present some basic and rudimentary notions about colour and what it is. Look in any textbook on colour and you’re almost certain to find a picture of the electromagnetic spectrum looking something like this:



It was Newton, of course, who famously studied the relationship between wavelength and colour. Light is a form of energy called electromagnetic radiation. Light can be characterised by its wavelength and our visual systems are sensitive to wavelengths in the approximate range 400-700nm (we’ll deal with the exact wavelength range later). So we call radiation in this range the visible spectrum or, more simply, light. In my diagram above the short wavelengths are on the right and the longer wavelengths on the left. So we might simplistically think that, for example, light at 400nm is blue or violet and that light at 700nm is red. It’s nowhere near as simple as this but it would do no harm to think that way for the present.

The spectrum above raises two interesting questions straight away however. The first is, why – since the wavelength of light varies continuously from about 400nm to about 700nm – do we see these specific and discrete colours? When I was at school I learned the mnenomic Richard Of York Gave Battle in Vain to remember the order of the colours in the spectrum. But why don’t we see a continuous range of colours – or, to be technically more precise – hues? The answer is something called categorical perception. However, just as interesting is my second question. Why do the two ends of the spectrum look rather similar. OK, red and violet are not the same. But certainly, red is closer to violet perceptually than it is, to say, green. And yet in wavelength terms red is closer to green! I’ll be returning to this issue of circularity of hue in a later post. However, if you would like to explore either of these phenomena yourself then I would encourage you to spend time looking at a rainbow. When sunlight strikes droplets of water in the air (this often happens on a sunny day after a rainstorm) the wavelengths separate (a process called refraction) and we see the visible spectrum. Newton achieved this by passing sunlight through a glass prism but the effect is the same, and equally enjoyable.

Interestingly, although Newton observed 7 colours when he separated white light with his glass prism, most scientists today agree that it is really only possible to discern 6 colours and that indigo cannot be distinguished from violet in the visible spectrum. Again, don’t take my word for it. Go out and look a rainbow now!!! The following relationships between colour and wavelength are often quoted:

Red —- 635-700nm

Orange —- 590-635nm

Yellow —- 560-590nm

Green —- 490-560nm

Blue —- 450-490nm

Violet —- 400-450nm

However, be very careful. Newton famously wrote that “to speak properly, the rays are not coloured”. Now, I wonder what he could have meant by that?

10 thoughts on “Colour 101

  1. Interesting question: “does colour exist”? As a rather facetious reply, I would state yes – consider the Obama campaign?

    One could posit the same question in response to the notion of ‘sound’.

    Good to see a blog devoted to both the scientific and subjective/ affective properties of this most elusive phenomenon.

    One final thought; if are said to see a white light when we die, does that mean that we are spinning at an incredible speed somewhere?



  2. I agree with Newton! The rays aren’t coloured, the rate of frequency and energy of the photons hitting the cones of the eyes and how we perceive it determines the colour. Ergo, We make the colour in our heads!

    I agree it’s six colours, though: Red, Green, Blue are Primary, Yellow, Cyan and Magenta are Secondary.

    My perception is that indigo and blue seem too close to me to be counted as separate colours, with cyan and violet either side.

    The wiki article on indigo is quite good when considering the ‘real’ perceived colour of the dye.

  3. Wow – nobody has suggested that before. You’re the first person to say the six colours are red, green, blue, yellow, cyan, and magenta. Yes, they are the primary and secondary colours of additive colour mixing but not the colours people see in the spectrum. Orange is always in people’s list – until now 🙂

    1. I think people mention orange because they’ve been taught that way. It’s a cultural thing. There are many websites that attempt to explain colour and of course, a diagram of the spectrum compared with a photograph of a spectrum shows up the inconsistencies between them. Beige, Crimson, Brown, Burnt Umber, etc. are also names for particular colours, or rather hues, but like orange they are not a fundamental colour. The six I mention are the result of direct or mixing direct wavelengths perceived by the eye. Or can be synthesized electronically without bias.

      The red yellow and blue painter’s circle is purely constructed by tradition. There is no scientific or aesthetic basis for that scheme.

  4. mmmm……

    This list of spectral colours is a bit too simple!

    Actually, in some cases (rather often!) we see only 3 colours in a rainbow:

    if the rainbow has some length ot project itself on a white wall for example, one will see, very close to the origin (a glass prism for example) violet, deep sky blue (“azzuro”), green, yellow, red, so something like 5 colours.

    Look at the part of this rainbow projected on the same wall, in the shadow, after 2-3 meters of distance – only 3 colours remain: Violet-ish, Green-ish, Red-ish

    Now, take a very narrow rectangular window that’s cut on thin cardboard and use it to look at segments of this rainbow: you will see hundreds of hues!
    Change the color of this piece of cardboard and you’ll see the hues differently….

    (sorry…) It’s also true that the illustration of the (visible) EM spectrum that you are using here is visually quite bad.


    1. Well, I did say that that list of colours and wavelengths is often quoted. It’s not from me. It’s on the wikipedia site I think. However, I do think that the list of colours and wavelengths is approximately accurate – that’s not to say that they can all be seen in the rainbow all of the time though.

      I agree it’s not a very nice image of the spectrum. I didn’t worry too much about that though – it was the concept I wanted to show. Of course the spectrum cannot be accurately displayed on an RGB device anyway. 🙂

      1. mmm…. The French wikipedia article “couleur” is horrible!
        haha… It’s so mixed-up & confused. English articles are usually a bit better, not conceptually but because we find more references, links and useful formulae there…

        I always have this feeling that minor articles are better, major articles like “colour” are systematically destroyed by clichés and failed pluridisciplinary attempts.


Leave a Reply

Your email address will not be published. Required fields are marked *