They get it when they molt (get ready to turn colors). First it will go black then it will turn a shade of color. As far as mine, they molt bits and pieces every few days, most of the time they will molt several times in the same areas each time turning a brighter or different color.
I agree with kobeclone that most of the improved coloration is the fish maturing however at the same time i do not want to deswade u on doing your water changes as they are very important in the longterm of the fishes health keep up the good work looks like u got a great devil.