Description of problem:
In keeping with the "minimal transitions" mantra, the initial plymouth field should be black, since most PC BIOSes are white text on black background, and clear the screen before handing off to the bootloader.
Technically this is not enough. You could have something else; Macs tend to have a white field, for example. But those would still have a flash either way. Eventually you'd like grub to infer this, pass it in the boot protocol to the kernel, have the kms driver inherit that for the initial screen clear... which, yeah, someday.
Fixed in rawhide.