Ornamental sweet potato (Ipomoea batatas), grown primarily for its purple, chartreuse, or variegated foliage, is a true sweet potato. As such, the tubers it forms are edible. However, their texture and flavor will not be as good as varieties developed specifically as a vegetable. You'd be better off with the garden types for stocking your kitchen pantry. Also, if you sprayed the ornamental type with pesticides not labeled for vegetables, the tubers should not be used as a food.