I assume you're offtanking a sister since main tanks dont need to worry about sisters.

While I do support your cause to add something to aid color blindness, you're going to have to rely on a FC that uses voice chat. Usually when sisters come we say "Red is the close one" Or, "Red is the left one". I know for duty finder that won't happen, but CORRECT ME IF I'M WRONG, does the sister have a "buff" that tells you which color shes giving to Garuda, so can you check the sister buffs? Again, I'm unsure if they have that.