This is a totally awesome answer. Red cabbage is known for its ability to indicate different pH levels. My sister-in-law and I discovered this while naturally dying Easter eggs, and my sister did an elementary school science fair project about it, and won! Basically it comes down to this, red cabbage, when in an alkaline (basic) solution turns blue, but when it is in an acidic solution, it turns purple and can even go as far as red.
Different soils have different pH levels. It’s why we planted our blue berries in a pot with only Mel’s Mix, which is more acidic than alkaline, while our native soil is very alkaline and would have produced terrible blue berries. The different soils around the world, and their pH levels, effect the color a red cabbage grows. In very acidic soil, a red cabbage is actually red; in neutral soil, a red cabbage is purple, and in alkaline soil, a red cabbage can go as far as turning green.