Are there colors humans can't see?

Red-green and yellow-blue are the so-called "forbidden colors." Composed of pairs of hues whose light frequencies automatically cancel each other out in the human eye, they're supposed to be impossible to see simultaneously. By some estimates, they can see about 100 million different colors or about 99 million more than we can! This is due largely to the fact that many tetrachromats have a fourth cone sensitive to the ultraviolet part of the spectrum, which is invisible to the human eye. Blue is the hardest color to see as more light energy is required for a full response from blue-violet cones.