Bananas are very sensitive to cool/cold temperatures. If bananas are exposed to temperatures below 55° for a sustained period of time the peel will start to turn a somewhat unappealing grey color. This happens to fruit that is bright green and fruit that is already quite yellow. Cold has the same effect no matter how ripe the fruit is before it gets cold. 

But don’t be fooled! The color of the skin does not mean the fruit on the inside is any less delicious. If the fruit is casting off a slight or high sweet scent you know it is ready to eat. 

Millions of pounds of perfectly edible bananas are composted/trashed by grocers because the skin isn’t the ideal color. This fruit is safe to eat and all the grey color tells you is that somewhere along the journey the fruit got exposed to too much cold air. So embrace the weird color of the skin and enjoy the tasty yum yum on the inside.