Have the lights been off in your tank for a while? Do you have light colored substate? While color change can be a sign of illness or stress, it can be for a number of other reasons too. Any other symtoms?
There was someone one here who had a fift tank where their mbubas which should be yellow turned black. Now thats a dramatic shift. It was just because the lights were out for a few days. Glad it colored back up for you.