Anthocyanins
Anthocyanins is the natural color in red cabbage that changes from pink to green depending on whether a liquid is acidic or basic.
Anthocyanins are like a strip of litmus paper hidden inside red cabbage. You drop red cabbage juice into a glass of vinegar, and it turns bright pink. You drop the same juice into a glass of baking soda water, and it turns blue-green. The juice is the detector — it changes color to tell you which kind of liquid it is sitting in.
Explaining anthocyanins by grade level
Red cabbage gets its purple color from something inside it. When you pour vinegar on cabbage juice, it turns pink. When you add baking soda water, it turns green. The color changes because the liquid is different each time. That purple stuff is a built-in color detector hiding in the cabbage.