We often get snap beans from the CSA (Community Sponsored Agriculture) that are either entirely purple or mottled purple. When cooked (at least when steamed or blanched), however, the purple color disappears and they turn to a bright green color. As far as I can tell, this happens with any purple podded snap bean.
Why does this happen? Are there any methods of cooking purple beans that retains their original color? Are there other types of vegetables that have the same color changing reaction when cooked?