My (so far...) one and only experience in dyeing was in high school drama. Our drama teacher had each of the "Roman crowd" dye our muslin tunics with "anything from a plant that you can find in your kitchen at home" (meaning, of course, our parents' kitchen).
My parents' kitchen had a glass jar of pickled beets. I don't remember any of us actually ever eating pickled beets, but the juice looked quite promising as a muslin dye! So I made sure it would be OK and, remembering what Mr. Petti had said, got the fabric "good and wet" first, then put the beet juice into a cookpot, followed by the wet fabric, followed by just enough water to cover the fabric.
Low simmer for maybe 20 minutes, followed by pouring everything into the stoppered-up sink. I left the cloth to sit until the water cooled down. Then I rinsed the muslin in cold water and asked Mom to dry the fabric in the dryer (although she had me wash all the dishes, vacuum the floors, etc., NO ONE was allowed to touch the washer or the dryer).
I am not fond of pink. But the color the pickled-beet juice produced was a wonderful, soft rose. I was almost sad to turn in my costume at the end of our run of Shakespeare's "Julius Caesar." I have no idea what the long-term staying power of the juice was, fortified as it was with vinegar.
There are several yarn dyers--some of them were at Stitches WEST--using only direct plant infusions/decoctions/etc. for their dyes, and the colors are amazing. Two whose names I can remember right off the bat are Alpenglow Yarn and Tactile Fiber Arts. They don't restrict themselves to foodstuffs, though.
What mordants are used on foodstuffs? Anything specific?