Sexual dichromatism is commonly studied in birds, fish and flying postage stamps (whoops I mean butterflies), but also occurs in various other species. This is a form of sexual dimorphism where the males and females differ in color. Sometimes the male is flashy, sometimes the female, and sometimes both in different flashy ways. Sexual dimorphism of body size is common in frogs, but differences in coloration are understudied and assumed to be relatively rare. Upon closer examination in a recent review from Rayna Bell and Kelly Zamudio (link here for the full text-it’s totally worth the read) a higher incidence of sexual dichromatism exists in frogs than previously thought. The authors explain that sexual dichromatism can be dynamic- usually appearing only during the mating season or ontogenetic- which appears at sexual maturation and persists. Interestingly, they find that 31 species from 9 families have dynamic dichromatism, while 92 species from 18 families have ontogenetic dichromatism.
This review brings up several interesting points, but specifically the pressures which may be acting on each type of dichromatism. In general, during dynamic dichromatism the male frog becomes more brightly colored to attract females only during the mating season and this would be subject to sexual selection. The females are choosing the best and brightest males. However, during ontogenetic dichromatism the male changes color and remains this brighter color. In this scenario, the male hopes to attract more females during the mating season, but when mating season is over bright coloration may make him an easier target for predators (not a problem in aposematic signals). In this case, it is sexual selection as well as natural selection that may be acting on ontogenetic dichromatism.
Read more in the paper about the evolution of sexual dichromatism from the link above.