craftgrrl | What's up with my sculpey?

So I bought some original Sculpey from Walmart the other day. I made something out of it yesterday and baked it. I baked it according to the directions – 275°F for 15 minutes (it was a thin sheet) – and it immediately burned. At least that's what I assume it did, since the white Sculpey turned this weird brownish purple color.

I took it out of the oven and let it cool for a while. It was still kind of bendable, so I stuck it back in the oven for even longer. It didn't change color much after that, but it's still slightly flexible. How hard is Sculpey supposed to get? Why did mine change color immediately instead of remaining white?