Off-white is not a special colour, it's typical of biological material that is neither pigmented nor unusually transparent. So it might be simply a non-evolved

Off-white is not a special colour, it's typical of biological material that is neither pigmented nor unusually transparent. So it might be simply a non-evolved default that is good enough to fool blind termites, themselves similarly off-white because they don't care about the visual appearance of their comrades.