I think a mixed palette of browns and tans will be your best bet, allowing for uniforms in different stages of age/disrepair and coming from various supply sources. I have used English Uniform as the darkest shade, with US Tan Earth and Orange Ochre for lighter tones. I tend to use a brown wash, then go over with the same shade lightened up with a bit of white or yellow (and highlight again if you have the time and inclination).
That has produced some pleasing results for both Whites and Reds. If it all looks too drab, the Whites can be livened up with one or two of the so-called 'coloured regiments' (I have just completed a Kornilov Shock Battalion in black, not that black lightens anything up, but there a few in white or light blue tunics you could go for), while officer regiments with mid blue (sorry, can't remember what I used) trousers and a yellow stripe work too.