Many, if not most, dottyback species change sex (or more correctly, are simultaneous hermaphrodites that allocate to one or other sex). The colour changes that are observed in many dottybacks is probably associated with sex change. However, I have been reluctant to link colour to sex for many of these. My observations are mostly on preserved museum specimens, and I often have little idea of their live coloration (unless a given specimen was photographed when freshly dead). In any case, because they are hermaphrodites, it's often difficult to decide on the sex of a specimen, because both gonad tissue types may be present. If I had to guess, I suspect the dark coloration is the male expression for P. elongatus and P. fuligifinis. More detailed studies are needed, however.
Tony