Re: blood mutation, I know there's at least one user who speculated that mutations like Karkat's are a brighter inversion of the root color, and made a whole chart for it. I can send the original post/user's blog name if anyone would be interested in it, as the chart they made shows the progression would fit Karkat's color perfectly (they also have other mutation designs that look AMAZING imo)

Ultimately though, mutations are just a fun gimmick to allow for differently-colored bloods for Fantrolls and the like, I feel like trying to set down a specific, canon color for each mutation would be kinda limiting!