Why is white tea white? The white tea wikipedia page is quite illuminating.

White tea is the uncured and unoxidized tea leaf. Like green, oolong and black tea, white tea comes from the Camellia sinensis plant. White tea is fast-dried, while green tea is roasted in an oven or pan (while kept moving for even curing). Oolong and black teas are oxidized before curing.

Also:

In hard times, very poor Chinese people would serve guests boiled water if they could not afford tea. Host and guest would refer to the water as “white tea” and act as if the tradition of serving guests tea had been carried out as usual. This usage is related to plain boiled water being called “white boiled water” in Chinese.