I wonder if this could be solved by just making the default emoji green or blue or something.

The advantage of a neutral color is that it can be whatever you want in your local theme. Back when they were images supplied by the respective forum/etc. instead of giant fonts that only mega-corporations can afford to maintain it was not uncommon to have emoticons styled and colored differently to match the site theme and/or subject matter instead of the standard yellow.

Skeeter is blue but represents black; Ice king is blue but almost certainly white. I don't know where Megamind fits in; and the Smurfs are almost certainly 'other'.

I think you're onto something.