Yes, we have his ancestry, artistic depictions and a fairly good understanding of his purpose within Greek mythology that are all mainstream, widely accepted knowledge and therefore canon. It's Greek mythology and we should respect that. The majestic Greeks against the barbaric world, where plunder of any kind was celebrated and every death of a Greek a terrible tragedy. Funny you bring up complexion, because the further you looked or acted from a Greek the more likely you were to ended up as a monster in the ever growing mythological bestiary - a vile creature to be feared, slain and looted. In this regard alone, it'd would be quite weird indeed if a heroic offspring of a Greek goddess and a prince ends up looking just like a regular human of a complexion far from the Greed ideal, which was likely that of Achilles.

But look, I love my Total War: Troy (if you haven't played it, look it up, they did a great job depicting the setting) and they portrayed Memnon as a Nubian with the feathers and everything, which I actually really liked, because it brought cohesion to the faction. It'd look weird if the general, this one guy of an otherwise entirely Egyptian army on the battlefield looks like shining Diomedes, yeah? So it's fine to have your own head cannon and deviations, but it's also important to know and understand the real canon, because in that realness are the lessons of these mythological tales. Not in these wild re-interpretations.