As far as I know they are tri color Leptosoma, two of the smaller ones turned a darker brownish color while my other ones are starting to color up in their fins.
It sounds like they are just going through their natural color change to me. Mine get darker occassionally too. I need to get more of these guys :) I really like them.
My next eartheater species is going to be Guianacara sphenozona. Very rare