I'd say it depends on the food . . . typically, with my cichlids it appears mainly brown or brownish-green . . . what you don't want to see (typically) is anything clear or whitish, and anything stringy or that doesn't drop from the fish
that seems to hold true for cichlids, anyway . . .
Yes agree on the food... My severums eat pink pellets and they're poo are pink other fish mistake it for worms... sometimes I grab some algae from another tank to their tank they eat it and they''re poo comes out greenish.