THE YELLOWING IS DONE BY NEITHER ENZYME NOR MICROBE
Yellow tea goes through the same fixing as green: a short fierce heating kills the leaf’s own enzyme. After that there is nothing left to oxidise with. And precisely then, when the reaction has become impossible, the leaf is heaped up warm and damp, covered with cloth or paper, and left alone.
What works under the cover is not life but the chemistry of warm water. Chlorophyll breaks down into pheophytins and the green turns to gold; catechins convert, and the grassy edge of green tea disappears without taking the sweetness with it. So yellow tea tastes like green tea with the edge removed, not like an oolong and not like a black.
The step costs anything from a few hours to a few days, and it leaves nothing visible: the dry leaf differs by a shade a buyer cannot verify. That is why the category nearly vanished in the twentieth century — it paid better to stop at green tea — and yellow remains the rarest of the six.