Are candy bloods the only blood color mutation?

The only one that we know of. The Hemospectrum seems to be limited to 12 colors- With Limebloods being extinct by the time Homestuck happens (And Karkat’s Candy Red being a mutation of the Lime Blood caste), and Fuchsias being limited to the current Heiress and Condy (As far as we know, maybe other Fuchsias are culled while the Heiress is still alive, and they’re still born).

That being said, given Limes mutate into Candy, there’s nothing really preventing other blood colors from appearing in the Hemospectrum through incredibly rare mutations. If you’re asking for validation on Pink Blooded Fantrolls, really, just do whatever you enjoy and don’t let people or canonicity bring you down! 

I should point out His Honorable Tyranny has Black Blood, and while he doesn’t look like a Lusus, it implies there’s creatures on Alternia with more than the 13 colors we know of, so… Who knows.