Hi Ralph.
I have already taken a look at plymouth. From my observation bg color is set during compile time so we need to check the code, too (similar to rhgb).
It's compiled into the plugin (or theme, if you want to call it that), isn't it? So we should be able - errm, I haven't looked at the code, I'm just assuming someone is able to - add a CentOS plugin to plymouth and make that one default. Or am I completely wrong there?
I am still not sure about. It seems that plymouth on RHE works a bit different as e.g. on Ubuntu (which is plugin based there). Maybe it's just an older version. Going to dig deeper and keep you updated.
Best Regards Marcus