I'm not an "expert" with convicts, or any fish for that matter. But many of my fish go thru the different shades as you have described. I think it has to do with the mood of the fish at any given time, whether it's stressed, angry, timid, or even if it's sleeping; sometimes I turn on the lights late at night and most of my fish have a pale tone to them. Of course a change in color could also mean a fish is sick, but if the color changes frequently, I wouldn't be too concerned, just keep a eye on him.