The six, and what defines each
GREEN: heated early to stop oxidation before it starts. WHITE: withered and dried, with no fixation and no deliberate rolling. YELLOW: green tea plus a smothering step that removes its grassy edge. OOLONG: bruised at the edges, oxidised partway, often roasted. BLACK: rolled and oxidised to completion. DARK: changed after manufacture by microbial activity, either wet-piled or aged for years.
The categories are not a scale
They are frequently drawn as a line from green through white and oolong to black, ordered by oxidation. That picture misleads in three places. White tea is not between green and oolong — it skips fixation entirely. Oolong is not a midpoint; the leaf is heavily oxidised at its edges and barely at all in the centre. Dark tea is not the end of the line; it is a different mechanism altogether, microbial rather than enzymatic.
What the categories do and do not predict
They predict the general shape of a flavour and the brewing approach reasonably well. They predict caffeine content poorly — leaf material, quantity and steeping matter far more than category, and matcha or gyokuro measure above many black teas. They predict quality not at all. And within a category the range can be enormous: Dan Cong and a roasted Wuyi rock tea are both oolong and share very little.
The naming is genuinely crossed between languages
English 'black tea' is Chinese hong cha, red tea, named for the liquor. Chinese hei cha, literally black tea, is what English calls dark tea. Neither language is wrong; they are naming different things. Most readers arrive looking for 'pu-erh', which is a Yunnan subset of dark tea rather than the category — Liu Bao and Fu Zhuan are dark teas and are not pu-erh.