Loss of colour to a fish (malawi mbuna) can be caused by a number of things.

A less dominant fish will often colour down, to avoid contact with a hyperdominant male in the same tank.

I actually find that males with no females will remain coloured up, and prioroties will change and the fish my slant towards possession of a particular part of the tank, over the search for a female.

The colouring down of fish can also be attributed towards a stress factor in the tank, weather it be illness or poor water perameters. Diet can also have an adverse effect too.

A good way to to test is to remove the fish, into a smaller tank and just watch it for a while to see if it regains its colour.

Certain species of mbuna can revert to change colour with reagrds to what sex they are.

Female auratus and lombardoi are known to change from female to male colours to fleet the aggressive advances of a sexually charged male.