Blue = green + violet.
Yellow = green + red.
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green + violet + red = white.
But when light is passed through two coloured glasses in succession, or reflected from two layers of coloured paints, there are prisoners to be made.
Blue passes green and violet only.
Yellow passes green and red only.
So violet is captured by yellow, and red by blue, green being free to pass on its way.
There is, then, a great difference between the mixing of colours, which evokes any tendency to antagonism, and the adding of colours under such conditions that they meet on equal terms. The first process happens, as we have seen, when a ray of light is passed through colours in succession; the second, when lights stream simultaneously on to an object. A white screen, being capable of reflecting any colour that falls on to it, will with equal readiness show green, red, violet, or a combination; but a substance that is in white light red, or green, or violet will capture any other colour. So that if for the white screen we substituted a red one, violet or green falling simultaneously, would yield blackness, because red takes both prisoners; if it were violet, green would be captured, and so on.