Why is it that purple potatoes retain their pretty color when cooked but purple string beans go all green?

Heat breaks apart the purple molecules on the surface of the bean, which exposes the green chlorophyll underneath, which is the exact opposite of what happens to leaves in the fall. (A similar chemical reaction happens when raw shrimp and lobster go from gray/green to pink/orange when cooked.) It's a pretty cool thing to show your kids or grandkids--they'll think you're way cool. Feed their brains while you feed them dinner.

and purple cauliflower also loses its color...