Two Colours

Color or colour (see spelling differences) is the visual perceptual property corresponding in humans to the categories called red, blue, yellow, green and others. Color derives from the spectrum of light (distribution of light power versus wavelength) interacting in the eye with the spectral sensitivities of the light receptors.

WHITE GRAY AND BLACK ARE BASIC COLOURS, THE MUST TO BE SOME OF ''REAL'' COLOURS RED, BLUE, YELLOW,......etc Add monotony of black and white picture a little color ! TWO COLOURS  mean WHITE GRAY AND BLACK are basic COLOUR, then one from real  colour like red, blue, yellow, etc.