The way we see
Anything we see is just a constant flow of photons - the particles of light, going through our pupils and being focused on the inner wall of our eyeballs. There are 3 different types of photo-sensitive cells responsible for our colour vision (cones): cells sensitive to red, green and blue light.
Light coming into our eyes is focused on the retina.
Therefore controlling these three primary "ingredients" of light enables us to control the whole range of visible colours, just about anything we can see. It also means, that we can describe any visible colour using only three values for each of the primary colours. A way in which we divide and descript colours is called a colour model. You will find out more about colour models on the next page.