Green is an unripe banana. They are a little hard and the peel is very thick.
A yellow banana with a little green at one end is the first stage of ripeness. The banana is not yet fully sweet, but it has begun to sweeten more fully.
A fully yellow banana is ripe, the peel has thinned a little and the banana is softer and sweeter.
A yellow banana with a couple brown spots is a full-ripe banana... this is its height of sweetness. These bananas also have the strongest banana flavor.
A banana that is more brown than yellow is over-ripe. Great for baking, but the banana inside is usually beginning to turn and is no longer very pretty to eat,... over-ripe bananas are super sweet and very soft, which is why they are perfect for making banana bread or cookies.