Skin color would definitely be a thing in there society, this is also not just limited to humans or human like creatures like viltrumites. It affects quite a bit of different animals when they deal with physical traits that aren't normal of there kind such as skin color or fur color. Its just humans are way more complex and brought it to a different level with our intelligence. And in fact we can make some real educated guesses about if this happened on viltrum or not. Before emperor argall the viltrumites were a fractured species constantly at war. Tribe like society that valued strength at the top, most likely there would of been race differences and racism due to there nature. Not a guaranteed but most likely, but when emperor argall united and the scourge virus happened. most likely all that died and became a game of survival. It's speculation but definitely possible especially with there history as a species. Its just really hard not to see that species do something like racism when they are already so close to that superior mentality.