The color changing mechanism for p. aur. is, as of yet, largely a mystery...some claim that good water conditions or lack of stress change them from black to gold, but really, that is more wishful thinking on their part, as such claims have yet to be proven---in fact, I've seen slate grey fish in the most pristine conditinos, and gold fellows in absolute wastebaskets, not to mention that golds are usually shipped in as such, and do not change a great deal along the way, so stress is out. However, they are capable of large color changes, most likely due to environmental cues--no one is absolutely sure what cues, though.