Not recommended to eat unless you are really hungry!!
This from Wikipedia: The flowers are used in food colouring. The leaves may be eaten cooked as well, but only as an emergency food
And this from Plants For A future: Tender young leaves - cooked as a vegetable. An emergency food, only eaten when all else fails. An edible crimson dye is obtained from the flowers. It is used for colouring cakes and jellies. The seed is crushed and used as a pepper substitute.
Better stick to dahlia tubers for an unusual edible I'd say.