What color is your substrate/background?
When my terror was in a tank with black substrate and dark background colors, she showed rich dark colors within seconds of putting her in that tank. When I moved her over to a tank with light colored substrate and a light background, she would change to a much lighter color.
I have another cichlid that does the exact same thing. Within seconds of putting her in the dark colored tank her colors went very dark, to the point that her striping was hard to see. I think it allows them to blend in with their environment, making it easier to both hunt and hide.
As long as it's not a juvenile terror, you can usually tell by the dorsal fin... males tend to have a longer 'tassle' at the tip of the fin, where on females it tends to be rounded.