Genuine henna is all natural. It's ground up leaves of the henna plant.
Great if you want super shine and red. It does not lighten your natural hair colour. It just glosses this beautiful super bright colour over your existing shade.
There ARE actually different types of hair colour that you might not be allergic to. It really depends on the allergy. I know that Goldwell colour uses a different type of pigment that some people can use when they are allergic to the other, more common, type of pigment. If you go to a salon that uses Goldwell and tell them about what you know about your allergy, maybe they can mix up a bit and do a text patch for you.
You might be sensitive to the ammonia or other ingredients, though, and you may be out of luck.
Personally I love henna and would never go back to permanent colour, but it's great for me because I want red hair. If you don't want the red you can use Indigo to get different shades of brown. Check out the henna for hair website.
Henna for Hair
oh doh.. I just saw that you wanted non permanent results.. Henna is permanent, though it doesn't damage the hair.
There are temporary and semi-permanent formulations in the Goldwell line.. I hope their pigments work for you!