They're not dithers, but I've found that keeping a few Cyrtocara moori with my labs has caused the labs to be much more visible. The moori are out most of the time, they are a nice blue color, and they really seems to draw out the labs. This arrangement also has the added benefit of allowing you to keep Malawi species together, avoiding the problems of different water preferences and the "grab bag" look of mixed tanks.
Also, note that ALL mbuna, by nature, will hide some of the time, whether there are other fish in the tank or not. That's their nature and the best you can hope to do is moderate it a bit.
HTH,
Jim