Saturation refers to the vibrancy and purity of a colour. The colours we see are made up from the three primary colours. A total absence of light – no red, green, or blue – of course is black. If full strengths of red, green, and blue there are equal, white is perceived (white light, as you will recall from prism experiments at school, contains all the colours). As grey lies between black and white, it must be made up of equal amounts of less-than-total red, and blue. It is the fact that the green, amounts are equal that makes grey ‘desaturated’ – neutral.