A flashy appearance doesn't always win a female; in fact, it can get you killed. While female guppies are lured by bright colors, predators are more likely to see, and attack, flashier male guppies. Indeed, in environments where predation is particularly intense, natural selection tends to favor dull-looking males--despite the fact that an unremarkable appearance slashes their mating opportunities.
Evolutionary biologists believe that females are attracted by bright colors because they suggest good genes and health. Since flashy males must contend with the constant threat of predation, bright colors may also indicate the presence of other desirable traits--like the ability to dodge fish that want to eat you.