Loupey, hawks have typical color patterns for each species, but some have variations in color called a morph. A rough-legged hawk is usually a light colored bird, but sometimes they are very dark in color... a dark morph. Red-tails can go either way and be either a dark, or light morph. My Sibley guide say they can even be albino.

I saw a red-tail in NJ once that was almost completely white, it had red-tail feathers and a few regular colored head feathers but that was it... coolest looking bird I've ever seen.