How to Make Natural Food ColouringAdd the red cabbage and water to a large pot and bring to a boil. Boil for about 15 minutes or until the water is very purple. Remove the cabbage with a slotted spoon (you can make soup with it, it's delish!) and continue to boil for 1 to 1 1/2 hours until it has reduced down to about 1/3 cup. Add in the baking soda a pinch at a time until it turns bright blue. Allow to cool.