I've heard of tea ware changing color when it gets hot. My daughter even has a mug on which a frog prince will appear whenever the mug gets hot.
Also, over at Phoenix Tea, we have little Rex the pixiu, who turns gold when boiling water is poured over his body.
I've never once heard of a tea that actually changed color! As it turns out Hong Bian gets darker with each degree of temperature loss. If you brew it with boiling water the tea liquor looks just like clear, plain water (it's also intensely bitter so I don't recommend it).
But when it's brewed at about 160 - 170° F it looks like this:
And by the time it cools to around 100° F it looks like this:
And if you put it in the fridge for a few hours, like I did here, it will come out looking like this:
So you're probably wondering... what does Hong Bian taste like? It's actually a pretty decent green tea. It delivers similar flavors, regardless of its current color. The taste is a little bit smoky but also fruity and sweet.