"From these primary colours the rest are derived in all their variety of chromatic effects by blending of them and by their presence in varying strengths. ...[M]ixture of white with black ...gives grey. ...[A] dusky black mixed with light gives crimson. ...[A] vivid bright violet is obtained from a blend of feeble sunlight with a thin dusky white. That is why the air sometimes looks purple at sunrise and sunset... So, too, the sea takes a purple hue when the waves rise so that one side of them is in shadow..."