Bear in mind that tanks take a terrible beating and are exposed to a harsh enviroment. If you are close to the "correct" base color, you are good to go. After weathering, it will look good.

If you ever happen to go past a military depot or base, note that the vehicles parked there were painted the same color to start with, but most seem to be slightly different shades after exposure to the elements. I work near Fort Irwin and often see strings of M1a1's and although they are all tan, there is alot of variety in the shade of tan on different tanks.

Russian armor green is certainly the best choice as a base color, but you could certainly lighten it slightly with white or darken it slightly to fit you taste or reflect your weathering. Looking at wartime pictures is a great source of inspiration. You have a fair amount of leeway, so have some fun with it