Because they are unoxidized, green teas keep their vital green color.
To prevent oxidization, the leaves are heat processed to eliminate the enzyme responsible for oxidization.
In China, this is generally done by roasting or pan-firing the leaves, while the Japanese generally accomplish this by steaming the leaves at a high temperature.

The definition of green tea in it’s simplest and generalized form is a tea that is not oxidized, with vegetal, grassy aromas, worldwide appreciated for the healthy, beneficial properties.
However, no tea is truly unoxidized because tea leaves begin to slowly wither and oxidize the moment they are plucked. For that reason, carefully controlled processes are applied to prevents the leaves from undergoing any additional oxidation, the so called poliphenoloxidase, preserving the green color of the leaves and halting the myriad of chemical reactions taking place as a result of oxidation. The most used form of green tea production are: heating the leaves shortly after plucking – a process called “kill-green” (from the Chinese word shaqing which means “killing the green”), typically done over a heated wok, in an oven, on a bamboo basket over charcoal or steaming (a merthod mostly used in Japan) or
sun-drying (oxidation is halted not by dehydration).
Several varieties of green tea exist, which differ in shapes, aromatic profile and taste.
To fully appreciate the flavor, better to prepare green tea with water at a temperature between 70° and 80°C.