I grew some purple potatoes this year. They were delicious potatoes that had purple skin, purple marbled with white insides, and the whole thing turned a deep purple when cooked. I was told that purple potatoes apparently originated from more mountainous regions, with the pigment to protect any potatoes near the surface of the soil from the sun due to the thinner atmosphere. The purple pigment is a sign of the higher content of healthy compounds in the potato!
I love how cool and whimsical this type of potato is, and it is used just the same as any other potato. Cooking with them makes my dinners more interesting visually.