what is wrong with their colors?

I honestly don't know.lol but I've wondered that myself. Maybe he did it to stand out from other shows, and it certainly has in my opinion. It's one of my favorite tv shows ever. I have the season box sets and still take them out every month or so.

I don't think the yellow color symbolizes anything. I'm not sure why they're yellow, though. There are black characters on the show like Dr. Hibbert and this family who are brown. Apu, who is Indian as a different shade of brown. Asian characters are a paler shade of yellow. And then, there are some pale characters who have a more realistic "white" tone to their skin. I'm so used to seeing the yellow characters that I don't even notice it anymore.