"Reddish hair" refers to hair that has a shade of red in it. It is a way to describe hair that is not fully red, but has a slight tint or hue of red color. Full definition
Fossil remains and anatomical reconstructions indicate that the typical Neandertal had a stocky muscular body with short forearms and legs, a large head with bony brow ridges and a brain slightly larger than ours, a jutting face with a large nose, and perhaps reddish hair and fair skin. (sciencemag.org)