‘Colour’ explains that colour is a construct of the brain, based on information supplied by the eyes. Objects in the world reflect different wavelengths of light to different extents, and our eyes are able to detect these differences and relay them to the brain. Humans have trichromatic vision and can see in colour because we have three types of retinal cone cells that respond to different parts of the spectrum — short, middle, and long-wavelength sensitive cone mechanisms. Colour blindness is caused by defects in these L, M, or S genes. Colour processing by retinal ganglion cells uses a ‘colour opponent’ structure. The behavioural difference of lights and pigments is also explained.