Colours are very simple, right? Everyone knows the rainbow is made of red, orange, yellow, green, blue, indigo and violet. But wait – where’s pink or purple? Well, there’s a bit more to it than we may initially realise.

Ok, let’s start with the basics then; what is colour? Colour is the perception of specific electromagnetic waves. If you know a bit of physics of optics, you’d be familiar with the fact that light behaves like a wave. The wavelengths of this light are what gives rise to colour. We may perceive light with wavelength between about 620nm and 750nm as red, green would have wavelengths between roughly 495nm and 570nm and so on. Of course, we are limited to seeing electromagnetic waves in the visible light spectrum, we can’t see ultraviolet, infrared, x-rays or any other electromagnetic waves outside that range. In a sense, you could refer to those wavelengths as colours we can’t see (there are some animals that can see colours outside of the visible light we know, but that’s a whole other topic).

Of course, things start to get more complicated the more things we consider. Going forward, it’ll be useful to understand how our eyes perceive colour; Our retinas consist of rods, that perceive brightness, and cones that perceive colour. Most people have 3 kinds of cones, each being sensitive to particular wavelengths. These consist of long, medium and short cones, which roughly correspond to red, green and blue wavelengths respectively. Colour blindness arises from a variation of one or more of these colour cones. There are also people who are tetrachromats. They have an extra kind of cone cell which allows them to distinguish colours much better.

This diagram above shows the normalised responsiveness of each human cone cell. It may be noted that there is greatest overlap between the medium and long cones, which indeed is where most people are most sensitive to seeing. Most eyes are best at distinguishing green colours.

So to see a colour like yellow, your red and green cones must be activated in certain amounts. This leads to an interesting illusion however; there is no way for your eyes to distinguish between pure yellow light and a combination of red and green light. They both activate your red and green cones and hence you perceive yellow. This is in fact how computer displays work. Screens can’t emit pure yellow or cyan light. Instead, they consist of pixels that can emit red, green or blue light and can “trick” our eyes into seeing many different colours.

We can now return to answer why pink doesn’t appear on a rainbow. There is no single wavelength of light that corresponds to pink, purple or magenta. Instead, such colours arise from your blue and red cones being activated. Your brain practically makes up those colours to make sense of your red and blue cones being activated without green. Indeed, if all cones are activated, you perceive white light and if none are, you perceive black.

Overall, the crucial thing to understand about colour is that it’s all perceptive. It is not an intrinsic property like mass, volume or chemical composition. It is subject to different conditions and the context in which we see a colour may completely change how we perceive it. There’s so much more that can be discussed, like primary colours, additive and subtractive colour mixing, optical illusions with colours and many more. I hope you now understand how pink is a pigment of your imagination.