I was curious about yellow tea also and according to Imperial Tea
Court Lew is right on the money. Mr. Fong says its very rare and
slightly oxidized.
BDB wrote:

From what I understand many teas labeled green are actually yellow tea
and (I'm guessing) don't have as many health benefits as green tea.

This is incorrect. Yellow tea is an uncommon type Chinese tea; it is
very similar to green tea in that it is not oxidized at all. The dry
leaf has a light yellow cast. Most of the teas labeled green are actually
green teas, although the infused liquor may often look yellow or
green/yellow.

I'm not sure about the "not oxidized at all". There's a lot of
mystery around yellow tea, but according to what I've read, it seems
to be dried more slowly than green tea, allowing for a marginal amount
of oxidation.

/Lew
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Lew Perin /
http://www.panix.com/~perin/babelcarp.html