I bought a red cabbage to use to try to dye an egg blue when I was doing my blue month - failed miserably. However, I did find it an interesting view when I chopped it in half.
@gailmmeek I found a blue egg cup in the cupboard and decided a nice, easy picture for my blue month would be a blue egg in the egg cup. If you boil the cabbage and add white wine vinegar and then leave the egg in the mixture it does turn blue but I could not get it to coat evenly - it just looked blotchy (in a bad way). It worked much better than the blue food colouring though.