Consider the image below:

The four grey patches in the centre of the four squares are phsyically the same. If seen in isolation (or against a common background) they would look the same. So are the grey colours the same or different? 

Some people would argue the two greys are really different (this is convincing because after all they look different). However, others would argue that they are the same colour – it’s just an illusion that they look different because of the background and context. Which is right? Actually, I think both are correct. The former would be talking about perceptual colour and the latter would be talking about physical colour. Part of the reason that colour is so complex and that there seem to be so many ideas that clash is that colour is not very well defined. When people talk about colour they are sometimes talking about different things. I am wondering whether it would be really helpful to develop more formally the ideas of physical and perceptual colour.

In case on your screen the greys look more similar that they do on my screen I append another example of colour contrast below.

In this example, there are only two physical colours (red and green) or three (if you include white). However, perceptually there are two reds and two greens.