You know those round cherry tomatoes that have the dark shoulders?  Yeah, those!  Well they are purple before they ripen to that lovely shade of red.  This pigment comes from a chemical compound known as Cyanidin.  If you have played with color on your printer you will be familiar with the term Cyan, but more commonly known as the color blue.  At a higher pH the cyanidin is more blue, the lower the pH the redder it becomes.  It is the compound that gives the color to most berries and also red cabbage and red onions.  In the health food stores, you can buy anti-oxidant supplements that contain anthocyanidins or you can just eat these gorgeous, delicious tomatoes.