The anthocyanins in purple sweet potatoes act as indicators and turn blue when exposed to alkali. Add the yellow of millet and it becomes green.

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

Blueberry trifle (the kind of British muffin, not steamed cake) sometimes turns gray-green because there is baking soda in the baking powder. It turns out that there is a cafe in my university whose specialty is green blueberry trifle.

If you cook purple sweet potato alone, you will get a charming emerald blue color like turquoise...