Kaiser Glass probably explained it best in a recent newsletter:
“All the colors are pretty stable at temperature except the red color. The red color is a metal oxide. It needs to hold that oxygen in order to give that luscious red color. When the metal oxide is heated in an oxygen poor atmosphere, it will give up its oxygen pretty readily to another element that wants it more. In our kilns that element is carbon. This comes from burning thinfire paper (organic binder) or any other organic material in the kiln.”
The cure is to vent the kiln to 1000F.