Try to imagine reddish green — not the dull brown you get when you mix the two pigments together, but rather a color that is somewhat like red and somewhat like green. Or, instead, try to picture yellowish blue — not green, but a hue similar to both yellow and blue.
Is your mind drawing a blank? These colors exist, but you most likely can’t see them. These “forbidden colors” are the subject of this interesting article by Natalie Wolchover over at Life’s Little Mysteries.