The tea friend asked: Why?

Indeed, the soup of many teas will darken after being left for a period of time. This is because the polyphenols in the tea soup are oxidized by oxygen in the water and air, producing dark-colored pigments.

Generally speaking, the color change is more noticeable in light-colored tea soups, while the soup of Black Tea or ripe Pu-erh tea is already red, so even if it darkens, it is not very obvious. Teas with a lighter degree of oxidation show more noticeable change than those with a heavier degree of oxidation, because the lightly oxidized tea soup contains more oxidizable polyphenols. Tea soup made from large-leaf variety fresh leaves changes color more noticeably than that made from medium and small-leaf varieties, also because the tea soup contains relatively richer polyphenols.

"So can we say that teas that easily change color have high polyphenol content and are therefore of better quality?" the tea friend asked.

"Of course not. The darkening of tea soup is the result of the oxidation of tea polyphenols, but there are many factors that promote the oxidation of polyphenols. For example, when brewing the same tea, if the iron ion content in the brewing water is high, the tea soup easily changes color; also, when the pH of the brewing water is high and alkaline, the tea soup also easily changes color. So, while the statement holds true when stated directly, reasoning backwards from it does not hold true..."

When studying tea, one encounters many situations that "hold true when stated directly, but not when reasoned backwards." Learning Tea Art is also a process of cultivating and exercising logical thinking—perhaps this can be considered an unexpected gain from studying tea.