The only source to really delve into it... was the Vis. Dictionary for the OT, Matt. It sort of tries too hard to associate specific rank to color though and of course doesn't go beyond the films.
The way I view it...
-Orange (In the Vis. Dic says it's Lt.) is any rank of Officer, from 1st Lt.'s on up.
-White (in the VD is Sgt.) is the varying levels of non-coms.
-Black is the grunts... Privates, Corporals, etc.
-Grey (debateable... Shots from the film clearly show that there is a pauldron that is NOT white, but not black by any means. It seems it's a very very light grey to me, but it's not white by a longshot simply by comparison of the trooper's armor) is what I've simply labeled as "specialist". Be it a comm specialist, engineer, etc.
-Yellow from Dark Forces2: Jedi Knight series is a heavy weapons specialist.
Of course though white, black, and orange are the only definites, and also there are troopers seen without any pauldron though it seems they're performing stunts.
I personally feel the pauldrons are a dumbed down method of identification for in the field or in civilian situations where average joes may need to speak to someone.