Hi,

First welcome to the forum. regarding the choice of colours then I would recommend to stick to the colours of red, blue, green... maybe combined with a litte White. These are the colours which I see most on medieval russians. They were quite colourful and I think looking at box covers for Zvezda Russian cavalry and infantry should give a good idea.
On the weapon issue, I would definitely give the Russian a sword (as he is stabbing) and remember to use a straight blade. For the enemy I would use a simple Mace/flail as these were very popular on the steppes.

Hope that this helps?