I have learnt this from a kids' science TV program recently. I found it amazing and wanted to share it with you too.
If you cook red cabbage, you will see that its water turns dark purple. If you pour some of that water onto a cup of plain tap water, the water turns purple. Then add some egg white and stir it well, the water will become blue. Then add some vinegar, and it will turn into pink/red. Add some baking soda, it will be blue again. Put some lemonade in it and it will amazingly turn into purple once again! (I hope I remembered them all right!)
It is incredible!
If you would like to learn more about the activity or the reasons behind the colour changes, here is one of the good sources for you.