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A brand of contact lenses is advertised as fighting crime by keeping away evil black eyes.
The villain dyes his eyes to a different color, and instantly becomes a good guy.
Zig Zagged: The suspicious looking character with black eyes is actually on the good side, but he turns out to be The Mole. However, as soon as he reveals this, he dyes his eyes. After which he goes back to the side of good because the black eye color actually shows that he is mind-controlled by the Big Bad, and it's rendered useless by the dye.
Averted: Who has black eyes and who doesn't has nothing to do with whether they're good or bad.
Enforced: We need a way for audiences to tell apart a villain from his heroic counterpart — let's give him black eyes so they can notice right away.
Lampshaded: "Fear me and my black eyes!"
Invoked: A villain makes his eyes black for the sole purpose of making himself look more evil.