Suusje

Correction: the "no fur" version can't explain the lighter parts - that's where the topic has started from.
But the "furry" version explains it easily: all skin of the Simpsons characters is pale, maybe its color is even close to our skin color. But we don't see it normally, because it's covered with the short yellow fur - covered everywhere. We can see it only in rare cases when a character loses a part of his/her fur or hair, like on the screenshot in the beginning of this topic. That also explains why Bart's, Lisa's and Maggie's hair have exactly the same color as their "skin" - their fur just have the same color on their heads as on their bodies, which is quite natural.