(Google search Guppylog for Anne Houde. Or get her book through
http://product.half.ebay.com/Sex-Color-and-Mate-Choice-in-Guppies_W0QQtgZinfoQQprZ297059
or order it through your public library.

She also noted, as have a lot of other studies, that where large predators (pike cichlids, Acara) cruised their waters, the male guppies were much plainer. The colorful ones got eaten. In fact the female guppies didn't respond to the mating dance and courtship very well in predator infested waters. There was the very real danger that the dance will bring a pike cichlid (Crenicichla species) down on her too. Sneak spawners might even have a better chance of mating!

Intriguingly the female guppies still preferred the colorful males, though the scale of colors was more subdued. Paul Loiselle published a field study on an Epiplatys bifaciatus. He noted that every morning the dominant male, the only member of the school really colored up, would swim out to his favorite weed patch or bit of hair algae. He would dance and boogie. Females, ready to spawn, would swim out and join him.

However from time to time a kingfisher, wading bird or other creature would pick off that relatively showy male. There would be a bit of a power struggle in the school of "pikelings"  and a newly colored up male would take his place by the algae of the day.

It may be that the brilliant colors impress the females with the health and strength of the males. They probably don't reason it out like humans would, but their stronger young have a greater survival potential.

Houde had already pointed out that females seem to know if a male has gill flukes. They will avoid him. In that case there is both the potential health of the fry to consider and the fact that they do not want to catch gill flukes from the diseased male!

And pouting lethargic fish are also targets for predators, avian ones anyway.