I can feel your pain. My Imperial Guard army is heavily based off the Kornilov Officers' Shock Regiment from late WWI and the RCW.
Honestly, don't bother with the vanilla Cadians if you want a Russian themed army. I converted mine heavily so they at least look Russian. I recommend buying or sculpting some heads, at minimum, to replace the horrendous Cadian helmet.
As for colours, I recommend painting the trousers/breeches dark (almost black) green and the tunics & caps either white (early war) or khaki (mid-late) for the basic infantry. Officers largely follow the same scheme with more gold and silver decorations, fancier shoulderboards, and a red stripe on their trousers. Shoulderboards were either blue or red for infantry depending on battalion.
Border guard regiments had almost an identical uniform as above, but their shoulderboard and trim colour was green.
Cossacks varied by Host, but all the steppe hosts had dark-blue breeches and cap crowns. The most common host troops you'd see in the east would be the Ussuri, Amur, and Transbaikal. The colours are as follows:
Ussuri: Yellow shoulderboards and cap band, green trim.
Amur: Green shoulderboards, yellow cap band and trim.
Transbaikal: Yellow shoulderboards, cap crown, and trim
*Note* Trim refers to any coloured trim on the uniforms AND shoulderboards.