Well I can't answer your question, but I have a distantly related one
myself. I was brewing some longjing at work and after maybe the fourth
or fifth brew I forgot about it in my yixing-style cup and went home
for the day. The color while I was drinking it was normal.

Yet when I came in the next morning, the tea was a dark brown color
with an oily sheen on the surface. It didn't smell strong or have a
particular strong flavor (I hesitantly tried a small sip before
throwing it out). I was trying to remember if I had brewed something
else that I didn't remember, but I am almost positive it was a fourth
or fifth brewing of the longjing I had brewed the day before. I can't
imagine that my few-month old yixing style cup was "seasoned" enough
to change the color that severely.

Has anyone had brewed tea dramatically change color before?

cha bing