I have had 3 separate L177 (only way I know to differentiate that this has only happened in the wide seamed large spotted varieties) that did this over the years. 2 arrived at that store partially changed and I was worried about illness so I took them home. The other one was my large specimen I'd had 6 years. He was not in the same tank with the others, nor exposed to them in any way. All of them kept the gold color for about 6 months, then went back, never to fade again to date. I had not heard of it in other species, but have had 2 other people tell me they had seen it in L177. I wish now I had taken pictures. I really did assume it was some sort of immunoresponse.
regarding H. luteus, this species is different. They change from dark to yellow/gold but don´t change back. I don´t think this is the same case as in all the others here.
Hypostomus auroguttatus has only be imported once by Pier. No idea how it looks now or where it is. Recently I´ve heard that they normally look dark, comparable to H. ternetzi. But from time to time some yellow ones seem to show up, even the species name "auroguttatus" leads us toward this. (auro = gold)
The white one from Mark looks like a Baryancistrus.
i have had it happen to medusa plecos.....ancistrus ranunculus....they were in an exceptionally dark tank, without Rio Negro colored water, and no light.....When i added a new bulb, they changed back....