Slice the cabbage and add it to a pot of boiling water. Let it boil for 10 minutes or until you can tell the water has turned purple. Remove the cabbage and set aside.
Add the ¼ tsp baking soda to the hot water. It will cause a chemical reaction that will change the color from purple to blue!
Let the water cool and store it in the fridge until you're ready to make the lemonade.
Pour the lemonade into the pitcher and add 2 or more Tbsp of the blue water to the lemonade until you're happy with the color. Add the lemons and ice. Serve chilled.