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EH. Not sure if I agree with that. The New Goblin was kind of an anomaly. Sandman and Venom still had their traditional colors and even the original Goblin was super cheesy.
I almost don't even want to count Nolan's films because the grey villains tend to refer to monsters, or big robots. TDK villains were able to have pretty dynamic colors. Brown and Blue for Scarecrow, purple, white, and green for The Joker. Even
Bane had his brown coat. And not like these villains had to have a lot of color anyway since they were villains.
Honestly I think it goes back to the X-Men movies. Though there, it applied to the heroes, not the villains. All the colorful costumes were kind of done away with in favor of the black leather suits because that was more "serious".
I can grant maybe people say the Nolan movies and got the wrong idea