Vincent Van Gogh: Selfportrait, 1889.

Look at the blue dot, blinking as less as possible for about twenty seconds. Then look at the white background. The orange dot is flashing before your very eyes. When you look at the orange background, the blue dot is flashing. For the colours of the rainbow, the eye forms the following pairs:

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We call them
complementary pairs

Two colours in the left painting are in a complementary contrast (contrast = the opposite): yellow and purple.