When I was in graduate school a century or so ago, I took a course in Fabric Design. One of the sections was on vegetable dyes. I have forgotten most of what was there (I still have the notes, but they are buried!). I do remember the beautiful pastel, soft (not harsh as aniline dye) yellow. The vegetable was onion, and the mordant (if memory serves) glauber's salt (sodium sulfate).

Good luck and above all, have fun with it!