The reason that Karkat’s blood is mutant isn’t because it’s bright. Equius has the same luminosity and he’s still considered a highblood. Karkat’s blood is mutant because the bright red color is on both the highblood and lowblood side of the spectrum.
It makes more sense really, since if karkat’s simultaneously the lowest a lowblood can get also the highest a highblood can get, it threaten to wreak havoc on the enitire hemospectrum. (he’d turn it into a hemocircle.)
Cheloneuniverse 