Impossible colors or forbidden colors are hues that cannot be perceived in ordinary viewing conditions from light that is a combination of various intensities of the various frequencies of visible light. Examples of impossible colors are bluish-yellow and reddish-green. This does not mean the muddy brown color created when mixing red and green paints, or the green color from mixing yellow and blue paints, but rather colors that appear to be similar to, for example, both red and green, or both yellow and blue. (Source)




