We've only seen Purplebloods like Gamzee and Chahut having red scleras; do you think that the scleras of the lower bloods can turn red too? Or is it a feature exclusive to Purplebloods only?

Given Terezi in this scene, it’d seem the reddening of the sclera are the equivalent of a Troll’s eyes going Bloodshot, so it’s likely a thing that happens to everyone.