Well, from a biological standpoint, their are multiple reasons in gender differences, including color. In birds for example, males tend to have bright, vibrant colors, as opposed to the rather drab females. Having different colors would allow for a means of gender display as well as gender recognition. This would be key in a society of Hippos (or Hippowdon), as Hippos live in herds of multiple females dominated by a combative, powerful male. This male, and hippo(wdon) society would benefit by being able to easily differentiate between a docile, welcome female and a violent, imposing male, as it would facilitate societal interactions and display. This is all my personal speculation regarding Hippowdon of course, but I hope this helps.