I love onion peel as a warm yellow dye for fabrics. It is extremely light and wash fast. You can use onion peel on unmordanted wool, or use with an alum based mordant on all fabrics. When you use onion with a post mordant of ferrous sulfate, you get amazing mossy greens.
Onion peel is obviously easy to get in the supermarket, but you need a lot, which is why I have found this extract which you use at 5% only. Dissolve the extract in warm water and heat well while stirring, add your mordanted (or unmordanted) fabrics to the dye bath and heat just below a simmer for about 40 minutes.
You can re-use the dyebath for lighter yellows.
Overdyed with indigo you will get the most beautiful greens, and with madder delicious orange shades.
 
Tech specs:
Active dye compounds: 
quercetin, isorhamnetin, and kaempferol.
Use at 5% WOF
100 grams of onion peel extract will dye 5 kilos of fiber.