Red is the rarest hair color, but to understand why that’s the case, you need a basic grasp of the science. There’s a lot more to it than the simple traits we learned about in high school biology.

Journal of Human Genetics - Genotype–phenotype associations and human eye color. ... a complete lack of pigment produces red eyes, and a small amount of pigment may produce violet eyes.

When we see animals' eyes in photos, nature films, flashlight beams, and headlights we see them shining back at us — but human eyes only glow bright red ...

Using human cells was key because you can't study how humans see color in a fly, or even a mouse. "Mice don't sense red," Johnston says. "They don't have these red-detecting cells.

Humans have three types of color-sensitive cones in their eyes -- attuned to red, green and blue light -- but birds have a fourth type, sensitive to ultraviolet light.