The reason that Okinawan sweet potatoes are purple is because of a pigment antioxidant that the Okinawan sweet potatoes contain.
That pigment antioxidant that the Okinawan Sweet Potatoes contain is known as anthocyanin which is the pigment that causes the Okinawan Sweet Potatoes to be purple.
The Okinawan Sweet Potatoes have a much higher content of antioxidants than other sweet potatoes but they all are healthy.
They are very nutritious and contain high levels of antioxidants as well as vitamins, minerals etc.
All those are essential to your health.