Because the four different types of tea (green tea, black tea, oolong tea, and white tea) all come from the same plant, what creates each tea's distinct color and flavor is the region where the tea plant grows and the way that the tea leaves are processed.
Freshly plucked tea leaves are spread on racks and exposed to warm air over a 12 to 18 hour period, which causes the tea leaf to lose about half its moisture. Withering makes the tea leaf more workable. If the tea leaf was rolled fresh, much of its watery juice would be squeezed and wasted and it wouldn't hold the twist or curl imparted by the rolling process.
After withering, the tea leaves are rolled in a machine that simulates rolling a tea leaf between the palms of the hand. The rolling process twists and curls the withered tea leaf to break open its cells, which releases the enzymes that aid the oxidation process. Additionally it gives the tea leaf shape and character.
Oxidization of Tea
The rolled tea leaves are then spread on racks in a cool, humid environment where the enzymes released during the rolling process can soak up oxygen. This step is also referred to as fermentation. The amount of time that the tea leaf is allowed to oxidize is based upon the type of tea that is being made. For example, black tea is allowed to oxidize longer than oolong tea. After oxidization, the tea leaf is typically bright gold or copper in color.
Once the desired level of oxidization is reached, the enzymes that react with oxygen must be destroyed in order stop "fermentation". The tea leaf is exposed to hot air that ranges from 200 F to 250 F. The firing process takes about half an hour and changes the leaf to its final color: black.