If they were all that colour, it would make them easier to see on the highway at night!
A few years ago I heard about a deer (in Pennsylvania, I think) that had a coat containing white hairs that caused it to show up brightly in headlights at night, but in daylight it looked brown. Someone suggested it was evolution at work, because so many deer get hit crossing roads in the dark. I have not seen any glow-in-the-dark deer myself.
