Strange, my tea turns deep red, almost brown, when left in a thermos for long. It tastes different, too (I tried it once). So, I don't drink the tea anymore when I notice a change in color.
A coworker told me the color change may be due to the continuous breakdown of the catechins present in the tea, and this process speeds up with high temperatures.
I'm not just so sure what happens to the tea when subjected to low temperature for a long time.
