That's normal. They communicate via colours, and she has nothing to say right now. She's unhappy. It's not because he's gone, but because she is a social fish and her own species isn't present. In time, she'll relax and get over the fear caused by the other fish. Maybe then, she'll get some colour.
If she is a he, then the colours are turned off because he doesn't want to attract any more attention. Showing colour is challenging the alpha.