Actually, as far as I know, the "look" of a Primarch is greatly mistake for a mythical warrior, or someone who should be "above all others." A Primarch doesn't have to stand out. He may be a greater version of a Space Marine, but he's still a Space Marine when the days through.
Also, Horus is the first Primarch to recieve Terminator Armor, it was a rare thing back then.
So that could be what Leman Russ really looked like, the only thing that made the Primarchs different from their followers was there incredible strength and wisdom... and in Russ's case, drunken'ness lol.
The only "warrior" whoever stood out was the Emperor himself, cause well, HE'S THE EMPEROR FOR CRYING OUT LOUD!
"Fetch me another plaything... this one seems to have broken..."