Things are even a bit more complex. Using DNA sequences it became clear that the species D. albisella from Hawaii, D. strasburgi in the Marquesas match the DNA evidence. D. trimaculatus from the Red Sea and Zanzibar, and presumably the entire (Western) Indian Ocean , forms a distinct clade, more different than the Pacific D. trimaculatus is from the other three Eastern relatives. Some of the D. trimaculatus and D. auripinnis from the Pacific mix together on the phylogenetic tree so they really are very similar despite the different colouration.
One of the phylogeny papers also stated "On the other hand, D. auripinnis coloration patterns (yellow lower body and fins) may either be reflective of recent speciation with little genetic divergence (or lineage sorting), or due to ecological adaptation to turbid waters (Randall & Allen 1977)." They don't explain the turbid waters logic but often fish have a light belly to stand out less against the bright surface when seen from below. If the turbid waters are yellowish then perhaps a yellow colouration of the lower body works better. However, I've never seen noticeable yellow water colouration near reefs except for algal blooms in the shallow lagoons behind a fringe reef. I don't find it very convincing but it is my best guess at this point.