Maybe it would help to think about this in a different way.
Putting greasepaint on your kid would not actually make him look like Geordi La Forge. He would look like a kid in greasepaint. If your kid really wanted to look just like Geordi La Forge he would have to wear a latex mask, or one of those horrible plastic injection masks – and even then he would look like a kid wearing a mask with Geordi’s face on it.
When we dress up in costume we know that we are not going to actually end up looking exactly like whomever. What we try to do is convey what we think are the important aspects of that person (which would obviously be Geordi’s eye-visor and a star trek uniform – you got those two things and anyone would know who you were meant to be). Suggesting that one would need to resort to greasepaint to convey who a character was who happened to have dark skin suggests that the colour of their skin is actually the most important thing about them. You can see why that’s problematic.
