Possibly the most readily available dye source I know, onion skins also produce a reliable color, depending upon which mordant and what kind of dye pot you use. It’s also nontoxic and environmentally safe. Just using alum mordant, you can get a variety of shades by varying the pot or kettle. My favorite is brass, which will yield a strong but lovely golden orange to rust, depending on how long you simmer the wool. Iron will produce a greener tinge. Glass or enamel will give a strong yellow, ranging from light to dark again depending upon time. In my experience the colors are fast and fade resistant.
I always save the yellow skins when using onion for cooking. If you can’t collect enough on your own, grocery store and restaurant personnel are often willing to let you have their abundant leavings.
I’ve read that red skins will produce a different color but have never used them.