I am building Tamiya's BT-7 Model 1935. Going well so far, but when it's near-built, I'll have to put the appropriate winter finish on it. I have no idea how to go about it, what color to use, how it was applied, etc.

It was whitewash.

Here is a video suggestion with Mig Jiminez

This will give you some help to get started but you can do it any way you want once you see the basic idea behind it.