The fronnies changing colour could very well be the difference in water going in. Unless its just from you having your arm, gravel vac, etc in the tank spooking them a bit. {cichfanatic}
Be very careful with water changes. Cichlids from Lake Tangy can be very fickle to changes in water.
As for the dark fronnie, are you sure it isn't getting harassed? As said earlier, dark fronnies like that, can mean stress.
For it to be one only too & a fronnie. Leads me to being picked on. Also by the way it is sitting in the back of the pot & high up, seems like it is trying to stay out of others way.
Frenchy