Why is green tea at stores brown?!

Japanese green tea is greener in color than most Chinese green tea, because of the way it is processed. This is true even of teas of roughly similar quality. An exception is roasted green tea, like Hojicha, which is dark brown and brews a very dark brown colored cup.

Some teas are better than others. Honestly, you may simply be comparing high-quality loose-leaf tea with lower-quality tea bags, and they're not really in the same category. Sometimes if green tea that is supposed to be brighter green or more yellowish in color is instead browner, it's a sign of lower-quality tea. However, other green teas naturally have a darker, more brownish color.

All green tea will turn brown over time as the tea polyphenals react to oxygen. It means it takes more time until comes to stores and is sold.