A Additive Colour 21 White light passing through a prism creates all the colours of the rainbow but only three colours are necessary to make up all the others. These are called the additive primary colours (red, green and blue). Equal quantities of red, green and blue (RGB) light always make a neutral tone: white if they are bright enough; black if they are all absent. RGB is the additive world of light; of photography, video, television and computer screens. RED