Variations
There are other plants that can be used to make pH indicators as well: red onion, apple skins, blueberries, grape skins, and plums. Experiment with different sources of pigment, to see which produce the best indicators.
Can you use your pH indicator to conduct other useful experiments? What about testing the acidity of rain on a smoggy day? Or the usefulness of various antacid brands in reducing acidity of a solution?
You can use an indicator to write secret messages. Just use full strength lemon juice to write an invisible message on paper and let the message dry. To reveal the message, paint indicator over the paper with a paint brush.
You can actually buy pH strips that tell you the pH of a solution. How accurate is the pH indicator you made using cabbage compared to store-bought pH strips? You can purchase pH strips from aquarium stores or an online vendor such as Amazon.