In Os Simpsons season four episode Lisa's First Word Rod is displayed as black-haired in one scene, but otherwise appears with his typical brown hair. I would have expected that the coloring errors are no longer present at this point in the show's production.
screenshot from the season 4 NTSC DVD
It's not as exciting as it sounds. There are a ton of Simpsons characters that have incorrectly black hair, especially in the dark.
For example, from The Springfield Connection (S6E23) has Chief Wiggum with black hair when he should have blue hair identical to Marge.
It's even called out as a persistent issue in IMDB:
https://www.imdb.com/title/tt0096697/goofs?item=gf0850439
Chief Wiggum's proper hair color is blue. A common animation error is for it to be black in certain scenes (this generally happens in night scenes, or other scenes in dark places.)
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