I mentioned it a couple pages ago, but the white uniforms in ANH are for the Imperial Security Bureau, or ISB: http://starwars.wikia.com/wiki/Imperial_Security_Bureau
Ah, right, I missed that somehow.
However it does seem that the white uniforms being part of the ISB is currently only in the Legends canon so I’m curious to see what Disney canonize them as. I guess that also means that the guy in RO isn’t a Grand Admiral, but something else.