I personally list grey as specialists, a rank between the black grunts (or pauldronless troopers), and white sergeants or squad leaders (Non-Coms). With of course orange being an officer's rank, especially since they're referred to as "sir".
I think grey is debateable. A picture exists of a trooper with a pauldron that COULD be white and really dirty, however the armor is white and there's a distinctly different shade/hue of color between the pauldron and the armor (and both are colored similarly, of course, with the same dirt/wear). Hasbro's really dark pauldrons seem incorrect regardless... The POTJ Sandie is who I'm talking about here. The Evolutions seems like the proper color, if grey does indeed exist, however it's really a tough one to call.
I go with it because, well, I have grey pauldroned Troopers in my collection so it's easy to just say, "Hey, these guys are specialists", and call it a day.