Funny enough, the Doms are about the only species I CAN'T buy that for.
Black Kryptonians, pink Khunds, or even the as yet unseen pale white Xudarians and mauve Korugarians...no problem with any of them. (Actually, both of those being easy colouring errors, we may very well have seen one of them...)
But the culture of the Dominators is monolithic (in a way that's actually mostly believable), virulently racist, and culls the herd HARD, so it's hard to believe the blue-greens could actually survive once the yellows became dominant. (Pardon the pun, not really intentional, but there's no way to avoid it without getting really overly complex.) Harder still to think one could make it in the higher castes.
(Hmm...unless the white caste marks denote something different than the red ones... Assuming the yellows didn't commit complete genocide against them, it could denote ranks within a slave population, or a parallel (likely lower-ranked) caste division...)
But, you're right, I'm really picking at a rather insignificant point because it manages to hit one of my personal points of geek...and you've got me pondering a whole other possible unexplored element of the Dominator society...
And I do give the colourist points for at least having her range from green to a greenish shade of blue, which could come from the same pigment cells as the yellow of the regular Doms.