Probably the most important thing to appreciate when thinking about colour and colour perception is that the way the human brain processes what the eyes see is very complex and has a huge influence on how we perceive colour that is distinct from what colours are, in terms of the light entering the eye.

In terms of light, an Individual photon has a frequency that relates to colour: lower frequency for red, higher for blue. Of course there isn’t just one photon, there are loads of them. If the mix of photons come in all frequencies in equal numbers, we call that “white”.

The human eye has three types of sensing cells that relate to colour, one type is most sensitive to lower frequencies, and sees red, one to middle frequencies, and sees green, and one for higher frequencies, and sees blue. They are not, however, narrowly sensitive, so their ranges overlap to some extent. If we get some red and some green, we perceive that as yellow. However, this can happen in two ways. One would be photons of a single frequency between red and yellow, that equally excite the red and green receptors, the other is a mix of red and green separately. The eye can not differentiate between these two cases.

The perception of colour is affected by how the brain handles what you see. The brain is inclined to presume the light around you is white. If the light around you is not white, the brain will adjust how it weights the intensity of the average light it is getting from each of the colour cells to make you think you see white, but the effect of this is the adjustment will alter how you perceive actual colours. If the ambient light is yellow (so less blue than true white), the brain will amplify the blue it gets to compensate, so you perceive the world as being more blue than it really is. This effect can persist for a time. There was a restaurant I used to go to that had pink lighting, and after spending a while there, when I went outside, for several minutes the whole world looked green.

if you have a light source of a certain colour, how you perceive it will depend not only on its colour but also how intense it is relative to all the other light. If it is very bright relative to the ambient light, it will trigger your brain’s white rebalancing, so it will seem more white to you.

Some of these brain perception effects in unusual light conditions can lead to strange illusion effects, a classic example being the photograph of the dress from a few years back that some people see as blue and black, and others as white and gold.