Conversando com TMZ, Matt Groening disse:
He was always yellow, and they painted him wrong once. 'At the time we didn't have enough to do retakes, so when there were glitches and mistakes it stayed that way. 'He was never 'black', it was an accident. (Source:dailymail.co.uk)
Então ele sempre deveria ser amarelo branco como o resto dos personagens, não preto. Agora, se você quer saber porque ele é amarelo; consulte abaixo a questão mencionada: -