So, when I said that Karkat was making a political statement with the symbol/color, this is what I was thinking of. There’s Redglare (right?) wearing the symbol (although I think right then it’s more of a secret) to signify that she’s a follower of the Signless/Sufferer, so maybe Karkat wearing the symbol is also a political statement, and not just a “Fuck you my blood is private” thing. 

I mean, he did say that he thinks the hemospectrum caste system is stupid, too. But that could also be a cover up for his candy apple red blood. Probably a chicken or the egg thing.