White would be sundering flatulence,
:rofl:
Anyway, the skin colour depends on which game you're playing.
http://www.imperial-library.info/bestiaries/battlespire_large_dremora-1.jpg http://www.imperial-library.info/bestiaries/morrowind_large_dremora-1.jpg http://www.imperial-library.info/bestiaries/oblivion_large_dremora-2.jpg
http://www.imperial-library.info/bestiaries/oblivion_large_xivilai-1.jpg http://www.imperial-library.info/bestiaries/battlespire_large_xivilai-1.jpg.
I have no idea why the skin colour differs from game to game. Different tribes perhaps?
And, as Gez said, Xivilai are not Dremora. To quote:
Xivilai, similar to the Dremora in every way, except they hate submission.
So, they're similar but not the same type of Daedroth.