Probably due to "mood swings"
brought on by the other ram and nothing to worry about. I have a male Mikrogeophagus altispinosus (Bolivian Ram) and he darkens now and then in response to environmental stimuli like other fish, feeding, water changes. With two in the aquarium there will be interaction, common in all fish but especially pronounced sometimes in cichlids.
There is no colour or pattern difference between male and female; the male has extended filaments on the caudal (tail) fin, top and bottom. If your fish are the same gender, they will interact more often with displays of dominance, particularly when one enters the "territory" of the other; a male/female pair will do this less often. This fish is believed to be solitary in its habitat.