One of my favorite dyestuffs is the onion skin. If you're new to natural dyeing, I think onion skins are a wonderful "starter" dye. They're easy to obtain, give good color even without a mordant, and (my favorite part) are something you might normally discard.
I use unbleached cotton muslin, silk organza, cotton floss, and undyed wool roving for dyeing. I mordant some in alum, leave some unmordanted, and use an iron overdip on others. What I've found with red onions is that the color can vary from batch to batch and exhaust to exhaust so it's a fun material with which to experiment. Well, provided you're not looking for an exact color of course.
Let's say you get excited about doing some dyeing and buy a whole load of onions. You know, like me. You've already made onion soup and various other onion dishes, so now what? You could always make onion jelly out of them. It's one of my favorites; great on pork, crackers with cream cheese, bagels...
...and it gives you a reason to keep buying onions.
Hope you're having a colorful day.