Whether it is green or yellow, it all depends on what you intend to do with them. When bananas turned deep yellow and brown speckles appear on the skin, they are fully ripe. The flesh becomes sweeter and fuller in flavor as they mature, so buy ripe ones if you want to eat them the same day. Otherwise choose those that are still green or just beginning to turn yellow if you intend to eath them later in the week as they will ripen quite quickly once you get them home.
When using bananas for cooking, especially for those savory dishes, use the fruit that is not completely ripe and still has partly green or pale yellow skin. That way will enable us to slice easier and the flavor is not so dominating. However, for the moist, high-fiber type of recipes, it is best to use fruit that is ripe.
** Asian Online Recipes