I've noticed this with blacks too. Especially yunnan gold. I'll be
distracted half way through a pot, and when I come back an hour or so later,
and the dark amber will have changed to almost coffee black.
I have noticed this for a while now but never thought to comment on it.
When I brew a green, when it's still hot/warm, the liquor is still whatever
it's fresh-brewed original color was (light green or blue-green or
whatever) but as the liquor cools off, it turns to more of a brown or a
straw color. It's really interesting. I can tell my tea is cooled off by