As birds become flightless, their feathers soon lose the barbule hooks that keep them in shape, becoming more hair-like; when flight performance is of no significance anymore, this can improve the insulating properties of the plumage. Nothing is known about the plumage color of Eremopezus; it was presumably not very gaudily colored as it had to avoid apex predators, but little else can be inferred. Perhaps most likely it had white, black or grey feathers with at least some eumelanin but little carotenoids and phaeomelanins, as usual among the "higher waterbirds" in general, and specifically those that inhabit similar habitat.
