What makes a purple potato purple?

They contain the same antioxidants found in blueberries- anthocyanin which gives blueberries their blue color and purple potatoes their purple color. The color that anthocyanin takes is based upon the pH.

A purple potato is a kind of potato that has purple skin and or flesh (sometimes called blue) with varieties being Purple Peruvian, Davis Purple, Eureka Purple, Fenton Blue, Purple Mountain, Blue Tomcat etc.