I successfully managed to change the background color for spinfinity. It's a PITA but works for me. I am sure there's a much more logical way to do it than how I did it but I'm not logical and have not figured it out yet:
First I hacked the spec file in the plymouth source rpm. I was wanting a solid black background. There were 3 background color related lines. Because I don't know what I'm doing I just changed all 3 in the %configure section.. Here's the old code and new code:
I then did an rpmbuild with the altered spec. That gave me a new source rpm and all the plymouth package rpms.
I then uninstalled all the currently installed plymouth packages. The uninstall also removed gdm, gdm-user-switch-applet, and mkinitrd.
Then I did an local install of all the newly created package rpms. Gdm was installed as a dependency.
Then I did a yum install of gdm-user-switch-applet and mkinitrd.
Then, as su in a terminal I ran this:
# plymouth-set-default-plugin spinfinity
As I said, there's probably a much smarter, abbreviated and simpler way to do this. I don't know it though and this worked for me.