Generally, bright colors are used to create a jolly atmosphere or to show a character's extrovert personality. However, sometimes they can hide an unexpected threat. And you may notice it only when it's too late. Why? Probably because you were distracted by all those vivid colors. Compare Light Is Not Good, contrast Dark Is Evil. Expect it to show up in a Stepford Suburbia, or even more probably in a Crap Saccharine World. Monster Clown is a typical character that fits this trope. Not related to smart and evil characters (you should look at Evil Genius or Wicked Cultured for this).