I just noticed that the colour of Groundskeeper Willie's Lip changes constanly, even within the same scene. For example:
Take a look at the colour of Willie's lip when he gave his views on Scottish independence:
Here you can see that the redness of his beard goes all the way around and presumably inside the mouth.
In this screencap however from the same clip, the far part of the lip now appears yellow, matching his skin. So which is it?
Some more examples:
With Beard
Without Beard
Rule one of The Simpsons, there is no consistency...