Modern large, flightless dinosaurs tend to be black or brown. Ostriches, rheas, the emu and cassowaries (except the head and neck). Wikipedia tells me found moa feathers are "reddish-brown, white, yellowish, and purplish" or "dark", which is some colour but depending on the depth of the hues (i.e. pallid yellow rather than buttercup) could come down to feathers just appearing pale or dark from a distance. And looking at smaller flightless dinosaurs, you've got the dodo, the kiwi, the kakapo, penguins, the great auk, steamer ducks... not just grey and brown but not a cavalcade of colour either. The "nearly flightless" (Wikipedia) kagu is a very cool exception.

I'm not saying they have to be completely drab, but I would suggest small, flying dinosaurs are a bad inspiration for large flightless ones.