Why are male fish nearly always more colourful than female fish?

This is related to the competition for mating opportunities among fish. There is still much debate among the scientists who work in this area as to whether the colours are a display of what 'excellent genes' the male possesses by avoiding parasitic infections or disease, or whether they indicate that he can afford to attract the attention of predators because they are in better physical shape. Either way, the theory is the more impressive the display, the more offspring the male will have.

"Hope is a duty from which palaeontologists are exempt."
David Quammen