I bet Kool-aid would be a good dye. I can see the color names now, great grape purple.... I'd wonder about the color fastness for things that aren't protected by a relatively mositure resistant layer like quills. I guess if you want to make it permanant you need to strap the material to be dyed to a three- year-old in white clothes who has been given a glass of kool-aid and told to stay clean:)
Onion skins do in fact produce a lovely pale yellow. I've never tried beet greens, but based on my aprons I imagine you could get a lovely pink from pickled beets, LOL.
Natural dyes tend to be more senstive to factors like pH and mineral content in your water, because unlike commerical dyes they contain no buffers or conditioners. (Iron in water will wreak havoac with many dyes, natural and otherwise. Dye molecules tend to be weak acids, this goes hand in hand with their fider binding ability, but it does cause them to be effected by their environment. And a number of the plant dyes used are also natural pH indicators, meaning that have pronouced color differences between the acid and base forms of the dye.) Actually, if you have hard water or on a well with lots of iron and sulfur, using bottled water for any dyeing. Natural dyestuffs also are more strongly influenced by the types of mordants used. I've seen yarn turned from yellow to army green by the use of iron sulfate mordant rather than a sodium chloride solution.
(Are you sorry you asked yet? :) )
Dying is a lot of fun (at least yarn and batik), but be aware that natural doesn't necessarily mean safe. Just looking at some of the plants listed on some of the sites, below, many are toxic and many are potential allergens. Strong colors often go hand in hand with stong defensive chemicals. Nature has been an incredibly inventive synthetic chemist and made most of the "really good" poisons. And some mordants are poisious and can burn the skin. So, use good ventilation, gloves and protective gear, get advice from an experienced artist, keep your dyes out of your kitchen, and treat all these substance with caution.
Links for natural dyes:
http://www.nativetech.org/ -- in the quills section. (They have a really good tip about the application of some kind of animal oil after treatment. The mink oil used for mink coats can help condition dry quills and furs.)
http://www.joyofhandspinning.com/natural-dyes.html
http://www.louet.com/nat_dyes.htm -- sells natural dyes stuffs
http://www.allfiberarts.com/cs/dyesnatural.htm -- as much as I hate to post a link with a link the Martha Stewart, this is a huge link list of natural dye sites
Sincerely,
OLChemist