Turmeric (powdered, dry) - yellow
Beetroot (juice) - red
Red cabbage (juice) - blue to red depending upon pH (add bicarbonate of soda / citric acid)
Green? Chlorophyll, which you might be able to extract from grass?
Black - powdered charcoal.
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A nice chlorophyll green is easily done with parsley + a bit of oil in the blender - just run through a fine sieve or cheesecloth when you're done.
Blueberries would make a fine purple, and they (or other berries) would likely contribute a beneficial flavor.
Thank you so much!!! Where do you find all of that stuff? At a whole food market I would assume?
Red cabbage is fairly easy to find (in the UK anyway) although it's actually a deep purple. If you boil it in water the water becomes purple. Drain and boil the water some more, it will reduce town to a small amount of blue/purple liquid.
Citric acid can be found in home-brew sections, or you could use lemon-juice.
Turmeric is a fairly common yellow spice, predominantly used in Asian cooking (e.g. "curry")
Both chlorophyll (the stuff that makes green plants green and vegetable carbon (charcoal) are authorised food additives, having European "E-numbers": E140 and E153.
Beetroot is a deep-red root vegetable, which I don't see fresh often (treat like red-cabbage). I believe that "beetroot Red" E162 is the same thing.
I suppose you could try using various fruit juices, all though this will probably have an effect on the taste of the frosting (which may be adverse to what you wanted)