It is obvious that the eye is not a mere passive receiver but actively produces its own colours after being stimulated by the light. It seems that not only beauty is in the eye of the beholder but also colour.The same phenomena appear in colour shadows. Where the colour of the light is red, the shadow appears green; where blue, orange; yellow, violet. In painting, red, yellow and blue are known as the primary colours. Where two primaries are mixed, as in water colour painting, we obtain the secondary colours, orange, green, violet.