Why ginger? No special reason that I know of. As a speculation it is possible that their ancestors passed through a population bottleneck in which a particular mutation (a gene for phaeomelanin instead of eumalanin in the hair, though not in the skin) got fixed. Alternatively, it might have been fixed by selection because in strong, UV-rich sunlight there are advantages to having light hair overlaying dark skin; the light hair reflects light and reduces heating, whereas the dark skin reduces UV damage to the skin. (I oversimplfy of course, but let's not write a book about this just at the moment!)

But I repeat: that is speculation.

As for camouflage, you might be surprised to see how difficult it can be to spot such creatures in their own environment. But be that as it may, there are complications. Orang utan red might not look very leafy to us, nor to another orang, from whom there is no great temptation to hide, but it might be quite a match to foliage for the eyes of a large predatory cat. They seem to have relatively poor colour vision. Anyway, it is not clear to me that camouflage plays a large part in practice in the life strategy of an orang.

As for hair in the tropical rainforests, it can sometimes get quite cold enough for a sedentary ape in the dark, to appreciate even a straw-like pelt like theirs. Possibly more relevant is the function in shedding rain when one morosely curls up in heavy rain. I am reminded indirectly, of an unfinished novel by Stevenson, in which he describes how the highlanders, if caught in a shower, would fold up their plaids in a dry place and sit it out. One's skin would dry faster than the plaid. 

Don't try it at home kids! Not in Scotland anyway, unless you are well acclimatised to showers that resemble horizontal stair rods, and feel ready to freeze on contact!