I think a better question is: "Why is a fruit named a color name at all?" It makes plenty enough sense to include a color name as a quanlifier, like "red beans", but what is "a red"?
Setting that aside aside, however, oranges aren't named for their colour, the colour was named after the fruit. Apparently we had no concept of orange in england until after we bastardised the spanish "naranja" (which became "a norange" before the n got misplaced). This is why a few things (most notably hair) are still referred to as "red" despite clearly not being.