First: I'm no expert on uniforms. Anyhow, I am pretty sure that these great coats weren't much of a different color than the general infantry ones.
Still, I found some pictures of dark grey great coats, as the parade uniform sported them. These almost look black, and you might dare to field an officer wearing it for whatever reason (- political commissar ?). Alpine units had (almost ?) black coats, too.
Uniforms changed their color immensely, as you can easily see on the German ones: The "field grey" was in fact a dark green - at least for a couple of weeks 'til it was bleached by sun and washing.
I doubt it was different on Soviet uniforms.
Here are some examples of different suits, even an officer's one. They pretty much look the same, and though hard to say due to b&w photos elsewhere, I think it was the same color, just a better and thicker material.
http://www.tridentmilitary.com/Soviet-W ... kages.html
I saw pictures of realistic re-enactment, showing no difference, too.
I hope I did help you ?
Gruss,
Marc.