Dealing with partial yellowing in a tan doll?

If it's just a few spots, you can try blushing reddish tones back onto the green parts. You can layer with sealant and pastel until you're happy with it, or do a bigger body-blushing job to give the whole body a bit of a refresher.

 

I agree with cobaltconduct, blushing the faded areas to match is probably the easiest. It's mostly fool proof, as it can simply removed and redone if it doesn't turn out right.
Blushing isn't very dificult and since it's likely mostly the red pigments that have broken down, it can probably be done in a single layer, or two, making it a fairly quick process.
The downside is that it does need to be redone every now and then. You can prolong the life of the blushing by minding how you handle the doll, choice of clothes and jewelry and what sealent you use, but it is never permanent.

It's not possible to dye a tan doll into pure blue. Nor is it possible to completely conceal an uneven colour. Dye is added to what is already there, it doesn't really cover anything. Dyeing blue over a beige colour will give anything between a desaturated dark blue to a greeninsh brown. The faded parts will likely still be paler and possibly a slightly brighter blue, since there is less to compete with the dye in those spots.

If it was me, I'd probably go the blushing route and if it turned out a little uneven, I'd try work it into the character of the doll. Not all humans have perfectly even skin colour, after all.