Re: why do purple beans turn green when they are cooked? And how did they get t

Interestingly plants, animals and indeed all forms of life have color.  
Yes, these colors are predetermined genetically, whether they are red 
cabbage, green lobsters or the green leaves we see change color in the 
fall.
Your purple beans are purple due to a pigment known as anthocyanins, 
responsible for that purple, reddish range of colors.  When cooking, a 
chemical change did take place.  What happened was the heat from the 
cooking broke apart the molecules on the surface of the bean and exposed 
the chlorophyll which is green.  In my other examples cooking destroys the 
lobsters' greenish 'coat' and uncovers its carotene underlayer, thus 
orange lobsters.  Nature plays similar tricks with leaves.  As the weather 
changes due to cooler nights and shorter days, the sugars stop providing 
nutrients to their leaves and the different pigments that lay under the 
green show up.  Such pigments as the anthocyanins that you noted in your 
purple beans.
Hope that helps.  Books that explain such chemical processes are:
The Cookbook Decoder by Arthur Grosser;  On Food and Cooking by Harold 
McGee.