I think you're drawing a false dichotomy here. You are assuming that aikijo must either be (a) just spearfighting with a stick in place of the spear or (b) a staff art which has nothing to do with spear and which therefore cannot be usefully informed by spear thinking. Why can't it be (c) a staff art heavily influenced by its founder's study of the spear, the understanding of which might therefore occasionally be aided by spear visualization?
You can call it a purple platypus if you want, but it's irrelevant to the matter under discussion, i.e., whether there is a functional reason to paint the ends of a jo so that you can tell one end from the other. Can you name a kata where the ends of the jo are differentiated, such that only one end can be used for a given technique? No? Then there's no need to mark a "this end" and a "that end", no matter what the antecedents may be.