How large were they? It might just have been the adult coloration. As my juvies are growing to adulthood, they are getting darker and more colorful with electric blue accents on the fins (haven't noticed on gills though).
Perhaps they were stressed at the lfs, and the stress of moving in to a different tank added to it? Perhaps they had some illness in conjunction? I really don't know. I've only had my kribs for a few months, and they are pretty young yet.