Across the animal kingdom, males can be brightly colored to attract females. Vervet monkeys also fall into this category, but only with their scrotums which are bright blue. However, this coloring comes with a catch: if a male falls in social standing, the color begins to fade. Thus, the females can easily identify the most dominant males and choose their mate.
Mas info http://pin.primate.wisc.edu/factsheets/entry/vervet
