If you are going for realistic colors anything and everything works. Why you might ask? Well you might. So the reality with uniforms on both sides is that the contractors who supplied the uniforms generally speaking were pretty cheap when it came to quality. Dyes ran when wet. They faded really fast and fell apart at an alarming rate. For the Confederacy there was little standardization. Dye was scarce and rarely available in large amounts. Colors of grey varied to such a degree as to be laughable.
That said For the Confederacy Try yellow grey, green grey, even brown grey. Using various shading and highlighting will get you an astonishing variety of colors. Actually it is quite fun to do so. For inspiration check out Keith Rocco's work. Fabulous paintings with impeccable research.