Hmmmmm....I was stationed at Polk from 1985 to 1986 and again from 1992 to 1995. Soil color? Dirt. Seriously, depended where on Polk and when. The weather there was like a lot of the "hole in the ground" places; if you don't like it now, wait a few minutes.
Honestly, were you to do a "normal" red-brown" color, you'd have it right. Mostly, I remember MUD during the rainy season (most of the year).....several tanks up to the turret buried; two or more 88's trying to pull 'em out...and having one heck of a time...my M577 covered, top to bottom, front to back (and me) in MUD......Or, doing 150 MPH in my buddie's Barracuda....sliding from Highway 171 across Highway 10. Those were the "fun" days.
Not much "specific" help, but hope it'll he'p ya some.
Mike