I know nothing about P's except what I've read. From my understanding when one is isolating himself and hiding as it appears in your pic's either he's sick or being picked on by the rest of the group. Also from what I understand during breeding or displaying territorial superiority their colors become more intense, which is the case with most fish and loss of color is usually an indication of disease or stress again as in most other fish. I'd thoroughly check for signs of disease and if he seems okay then divide the tank for a bit and hopefully he'll get his confidence back. Again I've never kept RBP's only a Sanchezi so I'm giving you my opinion which is definitely not well experienced.
