If you collect resin dolls, eventually they all start to yellow. Whatever light they are in, whether it is direct sunlight, indirect light, indoor lighting or, hell, just age - they all have an effect on the pigment of the resin and all - sadly - turn various stages of yellow. As the color pigments in resin begin to break down and fade the one color that seemingly doesn't is yellow. Red pigments fade the fastest and every brand, color and type will all react differently and fade at different rates. 

There is no way to avoid yellowing, it is a natural occurrence in the resin BUT there are ways to slow it down

The main way is to avoid sunlight. That doesn't mean you can't take your dolls outside for photoshoots (I'd be completely screwed if I couldn't play outside with my dolls!) Just don't leave them sitting around outside in the sun for hours or days. A small amount of sunlight is not going to immediately damage your dolls. Go play dammit, just do it smart!

Use a UV resistant sealant. There are a number of really great ones on the market, Mr Super Clear is probably the best known but ask around, there are new products out that I'm not familiar with - check with your favorite faceup artist, they will usually know what works best. Sealants can yellow, btw. That is easily fixable with a quick wipe and new faceup but the resin itself won't yellow.

Lots of other ways you can try to slow the yellowing process is to keep them in a room with the curtains drawn, you could dress them in clothing that completely covers them (though some dark clothes stain certain resins...), keep them all hidden away from everything and everyone in sealed boxes where nobody will ever see them (really, whats the point of having them then if you can't enjoy them?)

Oh - and keep them away from cigarette smoke. Just like your teeth, resin will absorb it and turn yellow too.

Look, yellowing is natural and inevitable - that doesn't mean you shouldn't enjoy your dolls. They are meant to be played with, customized, looked at, photographed and redressed. THAT is why you got into bjds to begin with, right?

Okay - SO - you have a doll that is yellowing what can you do?

WELL - funny you should ask. There are a few different options.

The sure fire way - you can sand off the yellowed resin with a high grit paper to remove the top layer and expose the original color.

The other way - paint it or dye it.  Read how to below.

(Note - the post below is copied from Den Of Angles. I have tried to locate the original author to ask permission to share this but have been unsuccessful. I think it's really valuable information and encourage you all to please visit the original post to read about others results and modifications. There is some really fantastic advice from a number of members and lots of comparison pics too.)

Author - Cymorill (DoA)
 

De-zombification Serum: Resin Color Restoration

I've posted a few experimental restoration threads in the past for mildly yellowed and mellowed resin, but I've finally perfected it in an easy-to-use method! This works for both sanded and unsanded parts! Please be aware that each part must be either unsanded (like hands and feet) or completely sanded if the seam lines have been removed to give an even tone! This method uses a special mixture of RIT dye to restore the red tones to resin that has faded and taken on that "zombie" hue.

Supplies:
1 bottle each of liquid (not powder) RIT dye according to the below formula
1 small glass bottle
1 eyedropper (or bottle with eyedropper top)
Water
Pan
Stove

To make "De-Zombification Serum" :
(Since the colors supplied by your local craft or fabric store can vary, I'll list a few different serum formulas)
Formula #1: Mix 1 part Scarlet Red, 2 parts Golden Yellow, and 3 parts Petal Pink in the small glass bottle. Close and shake until blended.
Formula #2: Mix 4 parts Rose Pink, 1 part Tan.

*Note: But my doll is already yellow! Why add yellow or tan dye? Because straight out red dye makes a doll look sunburned. The yellow tones in the resin also break down, but not to the extent that the red does and not as quickly. This mixture provides a flesh color, not that freshly-spanked-bottom blush.
**Another note: Interestingly, this serum looks like fresh blood. 

To use "De-Zombification Serum" :
** Make certain all doll parts are clean and free of MSC, blush and paint **
1. Measure out enough water to cover your doll parts and pour it into the pan.
2. Heat water to just under a boil (when it starts to get bubbles on the bottom but is not actively boiling)
3. Using the eyedropper, add 3 drops serum for every 1 cup of water and stir well.
4. Begin with a test piece. This should be an arm or leg piece, something smooth and easy to sand if it's left in the dye bath too long.
5. Submerge test piece in dye bath. Remove after 30 second and rinse thoroughly to check color. If the color is still too light, repeat until desired flesh tone is achieved. This will be your total soaking time! Soaking times will vary for each doll due to resin type and degree of yellowing. The time does not vary for sanded vs. unsanded parts of the same doll.
6. Submerge remaining doll parts in the bath, allowing them to soak for the established soaking time, stirring occasionally.
7. Remove pieces and rinse thoroughly in running water. Dry, and allow to cool.
8. If the color on a single piece is too dark or uneven (due to poor sanding) wipe the part with nailpolish remover and a cotton ball while the part is still warm. This will remove a little of the dye.
9. If your doll is too large or your pan is too small to soak all the parts at once, that's okay. BUT you must make a new bath for each batch of parts, as subsiquent batches will be lighter as the dye is used up. Dye time will be the same for each batch of the same doll.

That's it! This same method can also be used to color match hybrid parts.
The De-Zombification Serum can be saved and used again later. As with any dye project, be certain to observe the usual precautions. Careful not to get any of the serum on your hands or you might transfer it to you resinoyd and stain it.

Example: To restore a very faded Luts Kid Delf, I soak the parts for 1 1/2 minutes. To color match a very faded Kid Delf to a Minifee head, I soaked the body for 4 1/2 minutes.

Some comparison pictures
Non-sanded parts:

Sanded parts:

Do you have any tips or a solution that works for you - be sure to share it below so that others can benefit from your experiences. I'm still fairly new to all of this so I'm genuinely interested in learning!