Kids get colored braces as a fashion statement. They can choose their favorite colors or the colors of their favorite sports team. It makes getting braces a little less stressful for the child. It makes having braces "cool" for pre-teens. The color of the braces is the fashion and they don't have to match what you are wearing.
Colored braces don't necessarily make your wardrobe inflexible. They're are a good way to liven up what would be ordinarily be boring, which is why kids get them.
Because in our days, the color phenomena- it's in vogue, more colors you dress, the cooler you are, more unique braces you have, the more people want to know you.