|MadSci Network: Biochemistry|
There are obviously many foods that change color when cooked. This includes most meat products, which change from red or purple to gray-brown as the oxymyoglobin is denatured by heat. However, it is more likely your question is really, "What foods change color when warmed in the mouth?". I don't know of any foods that change color significantly over the range 10 C to 40 C. The most likely candidates would be fats and hydrogels (e.g., modified starches or gums with water). When these substances melt, they change from a white, translucent substance to clear. If you are trying to make demonstration project, try encapsulating food dye inside of a fat coating. When the fat melts, the dye would mix with saliva and make a strong color. There is a novelty trick of this type available. A special chewing gum that leaves the mouth black afterwards.
Try the links in the MadSci Library for more information on Biochemistry.