Hi rbcc (John), welcome to WebmasterWorld! So. you're hacking your boot screen.
osxml is an XML namespace. It defines a space where "text-mode-ui" means something. So yes, you do need to use it because XSLT is picky about namespaces; if you omit them, the sheet won't work.
RGBX and RGBI... hmmm. They aren't variables nor are they actual hex colors. They're probably just strings that refer to default values when rendered by Vista. If you're curious, try replacing them with something else, and see what happens. Worst case is you'll cause your PC to explode, shrapnel will pierce your skull, and the explosion will destroy your house and all that you love and deem holy. Or you might have to reboot Windows.
Make sure you take a backup and restore point first.