What makes an object ``purple'', ``pink'', or ``gray''? Color perception is unusual because it is purely the result of our visual physiology and neural structures, rather than something that can be measured in the physical world. In other words, ``It's all in your head.'' If two people have comparable color perception systems, then they can discuss colors using commonly agreed upon names while they perceive an object as having the same color. This contrasts other perception topics such as motion, depth, and scale, all of which correspond to measurable quantities in the surrounding world. The size of an object or the speed of its motion relative to some frame could be determined by instrumentation. Humans would be forced to agree on the numerical outcomes regardless of how their individual perceptual systems are functioning.