Springfield Elementary School is yellow in some episodes and
white in others, sometimes changing color multiple times in the same episode (later episodes corrected this continuity problem by making the school's official color yellow, and including a picture of the "old, white" school in Principal Skinner's office.)
When wearing her normal green dress, Marge wears a red necklace. However, often she is shown for a single frame or two wearing a white one (generally this happens in close-up shots of her head.) This happens in numerous episodes.
... almost everything. This is a comedy animation series, and while efforts are certainly made to create a vaguely consistent setting in which mostly consistent characters live and work, the emphasis is clearly on laughs. Individual episodes must be consistent within themselves, but rigid continuity between episodes is unnecessary. Indeed, additional humor has been wrung, during the life of the show, from the very fact that the episodes are inconsistent with each other. Goofs based on "In one episode they say... but then in another episode they say..." are not being accepted.
When Waylon Smithers makes his first appearance in The Simpsons: Homer's Odyssey, he has orange skin. He has the standard yellow flesh in all subsequent appearances. This was actually a change that the creators made knowingly. Smithers was mistakenly animated with the wrong color and was made an African-American by Gyorgyi Peluce, the color stylist. David Silverman has claimed that Smithers was always intended to be "Mr Burns' white sycophant," and the staff thought it "would be a bad idea to have a black subservient character" and so switched him to his intended color for the next episode The Simpsons: There's No Disgrace Like Home_.