Did you ever put clay into the oven looking bright and have it come out looking dark and dull? This sometimes happens to me with white clay, translucent and light yellows. The first thing I recommend is to give your oven a good cleaning, even if you never use it for anything but polymer clay. (A dedicated oven is recommended) I think that scum can build up on the inside of the oven and sometimes discolor light clay and affect the temperature. You’d be surprised what a difference a clean oven can make.
But what to do if your clay is discolored or even burned slightly? Before you throw it away, try bleaching it. I have used bleach on black and white canes. The white got whiter and the black stayed black. I don’t have any pictures of those. A few weeks ago, however, I made some earrings with lots of Premo zinc yellow and they came out looking dark and dull. I put them in a glass, covered them with bleach and let them sit for about a week. Look at the before and after pictures below. The bleach seems to lighten light colored clay but not to wash out dark clay. Try it yourself and see what you think.
Since I posted this, a person wondered if soaking baked polymer in bleach might weaken it. The answer is, “Maybe.” My friend Terri Powell who is a chemist and a polymer clay artist said, ” I don’t know the answer to your bleach question for sure, but my suspicion is yes. You’re talking about putting plastic in acid for an extended period of time. It’s the extended period of time that I think might be the problem–I’m sure just a wash probably wouldn’t be too bad, but the prolonged contact could be problematic. On the flip side, you could just be cleaning off the surface layer of yuck, and not doing much damage to the main body of the structure. I would do an experiment. Take two similar pieces, bleach one, and then try to bend/break both. That might give you some clues. “