Usually dark colors indicate stress or they're really cold...Or if they're basking it's normal for them to be dark as well. It helps them absorb the UVB rays.
When my chameleon is cold he turns dark blue with black bars. Haven't seen him like that ever since he started to learn to bask before sleeping and falling asleep in his basking spot.
Make sure your temps are on point.
Also Please fill out the Ask For Help form that is linked below so that other members can see how you're housing your chameleon. The more detailed info you give us the better help you'll receive from members.