It is my understanding that the color of olive oil is not an indicator of quality.
Green olive oils (the green is chlorophyl) are pressed from green (unripened) olives while yellow oils are pressed from ripe olives. In fact, olive oil producers can press some leaves in with the olives to give their oils a greenish tint.
The best advice is to taste it and buy what you like. Some olive oils will have a more pronounced olive taste. Some (green ones) will have grassy overtones. You may or may not like the greenish ones best.
"If you want to make an apple pie from scratch, you must first create the universe." -Carl Sagan