Not really a problem. More an annoyance.
I am running Photo Mechanic version 5.0 build 17338 (df27633) and I run OS X 10.11.3 on my iMac.
I have Photoshop CS 6 set in Photo Mechanic as my default editor for all common image files including RAW files.
When I open an Olympus RAW file (ORF) or a Canon RAW file (CR2) from Photo Mechanic it opens in Adobe Camera Raw as I would expect it to. However behind the ACR window there appears a Photoshop window that is either grey or white. What can I do to ensure that this background window is consistently the same colour, preferably grey.