If your eyes mix complementary colors, you see white

Don’t confuse complementary colors on a paint color wheel with those on a light color wheel. In the first case, if you mix two opposite colors you’ll get something brownish, as adding more pigments signifies that more colors are absorbed. A light color wheel is a different story: your eyes perceive them in combination, so you get white. The following video illustrates how it works. The red screen and the cyan screen are switched back and forth quickly, and you seem to see white between the shots. However, if you pause the video you’ll find the screen to be either red or cyan (this may not work on mobile devices).