I have noticed that the whites of the eyes do yellow over time. However, mine haven't turned super dark or anything. They tend to turn a light cream color.
I have noticed a few of mine with white scleras have yellowed over time. I've had the same experience as @Angelesk where they turn cream white in color. In my opinion, they look a bit more like natural eye whites this way, since human eyes tend to be off-white anyways.
The DIY resin eyes I made have yellowed if I left them on my crafting desk too long. The desk gets a lot of sunlight since I hate working under artificial lights. The professionally made ones haven't yellowed yet, but that's probably also because I do not leave them out as much. The room where I keep my dolls doesn't get a lot of sunlight on account of curtains and the window facing a wall.
Some of my eyes turned yellow -- the white parts of the eyes. Some didn't. I cast resin jewellery -- it seems to depand of particular resin and dye, even. Some yellow more, some are more UV resistant in my experience. Eye artists don't always write what kind of resin they use.
Hi!^^ If you like, I have recently posted about my experience concerning resin eye yellowing here (a bit too much to repeat it here word for word). Unfortunately my experience was rather negative, though. Generally speaking you are at the mercy of the eye maker, wether they buy the more expensive but better quality resin with added UV protection for example. Doll makers have been conscious of this problem for a while now and often choose their material accordingly - some eye makers not so much, amateur and professional alike. Sometimes makers use different resin material for different batches, so the yellowing isn't even consistent. In the end the old truth still holds: All resin yellows over time, only some yellows faster. So you can either try your luck or stay away from resin eyes if you'd like something that will surely last... Btw, some silicone used for eyes yellows as well, at that (hello, Masterpiece eyes!)
