1. Buy a purple cabbage and boil it in water. After cooking, remove the leaves and leave the soup. This water is the natural pigment.

2. Blueberries in fruits and blue cabbage in vegetables are also called red cabbage, but they turn dark blue when cooked.

3. Add a little baking soda to the purple cabbage juice and observe the color until the desired blue color appears.

About natural blue pigments:

1. Blue and green natural pigments are difficult to obtain, and there is currently no natural blue color approved by the FDA.

2. The phycocyanin that is only allowed to be used in Japan is blue. But the cost is high, estimated to be 200 times that of synthetic pigments allowed by the FDA.