Resin is going to change color, no matter what.  Some will show this more than others, depending on the mix. Even under "museum conditions", it is going to change over time. UV sprays and retarders will slow it down, but that's it. Brignt light, fluorescent light, and sunlight can speed it up. Tan and French resin are more likely to turn faster than "regular" resin. Red colors are more fugitive than most other colors (think about faded red vehicles, and kids' plastic toys that have been left outside!) so the red in the tan and pinker dolls will fade first making the color shift so that they appear more yellow, gray, or green depending on the color mix. And keep in mind that while resin is expensive because of the difficulty in the molding process, resin is just a form of plastic, which does break down over time.

A tan (or other colored doll) can be made to appear more even by carefully blushing them using pastels. Even the face-up can be done without changing the original one other than having to re-do the gloss. A full body blush means that the doll cannot be handled as much though as it is easy to scratch it on the body.

Here is a tan Dollshe Hound that had faded to a grayish color on the front side; it has been taken apart and cleaned.

This is what he looked like after he was blushed to even out his skin color; note that the face-up was not removed for this.  He was sprayed with Mr. Super Clear UV Cut and blushed using Schminke Pastels, and glossed on his lips, around the eyes, and on his nails using acrylic varnish.