Are trolls naturally red-green colorblind? This only came up while looking through the extended zodiac and trying to understand why karkat's sign is in the lime column.

We know nothing about Trolls’ sight cone thingies and how they take light, but I mean given their eyes seem quite sensitive to light, there may be something about them we don’t know about. 

That being said, the Cancer Symbol being Lime just proves that the Vantases’ mutant color is a deviation from Limes, as was already popularly theorized due to their position on the Zodiac. There’s not much more to it, though- It’s the Sign of the Signless, of a Troll who never had a Sign, and who adopted, after death, the symbol of his shackles. Maybe Trolls do know Candy Red is a mutation of Lime, maybe there’s some association with Cherub stuff that definitely influenced Alternia, or maybe it’s just Hussie confirming popular fanon cheekily.