The anthocyanin of purple sweet potato has the function of indicator and turns blue when it meets alkali. Add the yellow of millet, and it is green.

Did you put baking soda in it? If not, water may be alkaline.

Blueberry muffins (English muffins, not cakes) sometimes turn gray-green because there is baking soda in the baking powder. I used to have a cafe in college, and the specialty was green blueberry muffins.

If you cook purple potatoes alone, you will get a charming emerald blue …