As greene said, most of the flowers I've seen on the ornamental sweet potato Ipomoea batatas varieties are pink or pale mauve with a dark center sometimes. Also, the flowers are small and not very numerous. It really is a foliage plant and groundcover, not one you'd grow for flowers. If you click on Plant Database in the column to your left, and type in "sweet potato vine" there are pictures of all those that greene mentions above and more.
Are you sure the one you saw was not a Morning Glory? They're closely related. Some of them have gorgeous purple flowers, as well as lots of other colors from white and pink to the bluest blue. If you want an easy, vigorous vine for flowers, I'd grow Morning Glory.
