I've noticed that there is a difference in the yellowness between Chinese style (roasted) green tea and Japanese style (steamed) green tea.
The former is a rich, thick, golden color. The later is a pale yellow, with a hint of green......green-green tea being steeped to many times.
My local co-op sells bulk sencha from Frontier Herbs, which despite being sencha comes out with that faded, yellowish color. Some cheaper boxed steamed green tea I've had comes out green, but it only lasts for a few seconds.
I'm guessing that steamed green tea leaves are similar to green vegetables. The older they are the less green their stock will be and from things I have been reading tea can sit in warehouses for long periods of time even before it makes it to a shelf.
Anyway, I have my o-cha tea bags which are green-green, with that nice fresh grassy smell for at least two-three steepings.
I'm being cheap and have gone to 4 some I am drinking yellow.
A new Japanese friend of mine told she saves that used up tea to put in a jug of water water in the fridge to use as a sports drink at the gym.