Hmmm.
Okay, firstly dyeing wool is easy, and even on it the onions will fade out to buttery yellow.
Turmeric is better but you need a heck of a lot for a cotton tarp.
Basically these three dyes are the safe in class ones for schoolkids.....and you don't need to boil the wool in the onion liquor, just add it when the liquid is ready and leave it sitting in a jar until the colour is as strong as you like........nicely ticks all the health and safety boxes
and the kids get a result
Works on nylon and on paper too.
If you are dyeing a tarp, might I suggest that you have a go with a substantive dye, one that needs no mordant, like madder roots, tea, coffee, if you really want a dye and not a stain.
Thanks for posting the link, interesting to see.
cheers,
Toddy