Purple sweet potato contains a lot of anthocyanin, which is a water-soluble pigment that can change color with the acidity and alkalinity of cell fluid. Cell sap is red when it is acidic, and blue when it is alkaline. It is normal for people to turn purple or blue when they meet water. That's because saliva dissolves purple pigment. This natural water-soluble pigment is harmless to human body. Anthocyanin is extremely unstable, red in acid, blue in alkali and purple in neutral.