With green tea, the natural oxidation process needs to be halted soon after the leaf is picked.
Once the tea leaves are picked they are then laid out to wither then, but before the oxidation process really starts, the leaves are either steamed ( Japan ) or pan -fired ( China ) before being rolled . This is done to prevent the veins in the leaf breaking before rolling so that minimal oxidation of the leaf occurs.
The leaf is then dried and will have retained most of their colour, kept most of their natural nutrients and are generally, still low caffeine.
Some green teas will have been allowed to oxidise more than others, essentially the greener your green tea leaves look, the less they have been oxidised. As with white tea we are looking at quite a light colour brew, slightly greener tasting, a little more grassy.