Why call it RED tailed hawk?

The direction and quality of light is of paramount importance to photography. It defines everything about depth with light/shadow. It also controls color saturation/rendering.

This guy has been hanging around my backyard for about 2 months now, circling the property a few times per day. For this picture, I was happy with the richness of the red in his tail. This will keep me from accidentally calling him a blue-tailed hawk, or Bob or whatever.