Conceptually with the advent of the NT, seeing as how the pauldrons indicated ARC training and the color was based on organization, one would presume that these were specially ARC trained troopers (Vader's best!). I always preferred the theory that color was branch.
However we have the anecdote that when Lucas saw the sandtroopers he asked the prop guys how he could tell what rank they were, and thus were born the pauldrons.
So, even though what we see on screen percent wise and also via the New Trilogy doesn't bear it out, the original intent and current LFL marketing says they are rank based, does it not?
I think having black/white be corporal/sergeant or vice versa is fine, with no pauldron being a basic grunt trooper.
In other words, in my verbose style, ignore my previous post.