There are several different topics being discussed here: deer vision for blue, deer vision for hunter orange, and deer vision for UV. I'd have to dig to find them, but have read studies showing that deer (the entire family, including elk) appear to distinguish bright blue, i.e. "royal" blue esp. in large shiny garments and low light, like rainsuits. Duller and darker blues don't elicit the same response. Bears too seem capable of distinguishing royal blue, according to both studies and anecdotal reports from field biologists and others.
John N: Deer tails are white, and elk rumps are light, for what Dr. Geist and other experts refer to as "herd cohesion." You're feeding or running through a darkish forest and want to be able to stay with the rest of the group without vocalizing or watching them constantly, so you just glance at the beige rump or white tails in front of you. Of course, this basic evolutionary adaptation has been further refined in such as whitetails and pronghorns (not in the deer family) to signal various stages of alert and alarm, and of course to say to bowhunters, "Adios sucker! You lose again!" dave