Have you ever figure out that the lemon acid juice together with some blueberry juice (berry) changes blueberry colour into red? On the contrary, the alkaline sodium bicarbonate turns juice's colour into blue.

Design you own experiment:

Pour some blueberry juice into a pot or on a white dish, put in a pinch of bicarbonate and... can you see? Blueberry colour has become blue. Add some drops of lemon juice and the colour'll come back red!