Mexican Hats are included in the "Dyes from American Native Plants" book, and I have begun harvesting some for dye.
I have separated the various parts - stems from leaves and flowers. I put the flowers in the freezer - this is recommended by India Flint as a way of getting them to release their color better.
This patch of Wild Bergamot grows up on the hillside above my house. It is referenced in the book and so I think I will harvest some for a dyeing trial.