I think I unintentionally blurred together two separate things.

Citrus can be ripe and still be coloured green. Ethylene is used to make them orange, as they look more appealing to buyers that way.

Green bananas on the other hand are just not ripe. Ethylene is still used here, but to "kickoff" the fruit's ripening process - in just a few days it becomes yellow and ripe.

There's many things that release ethylene naturally when ripening, like tomatoes, apple, kiwi, ... These need to be kept away from other sensitive produce (lettuce, spinach, broccoli, cauliflower, ...) as they'll start looking "nasty" and lower their shelf life.

Thank you for that. It’s always nice to be “corrected” by a stranger who has no idea what they’re talking about.

Having had both mandarin and orange trees in back gardens in Brazil, I stand by my confidence.

From a link posted elsewhere in this thread:

When they’re expose to temperatures below 55°F (12.7°C) for long enough, the green chlorophyll breaks down and the orange carotenoids surfaces in a process called “degreening.”

Are you confidently suggesting that in Brazil it doesn’t get colder than 12.7°C? I have a land plot on the moon to sell you. Or, if you prefer to be educated instead, I can point you to some lovely mountainous places to visit in Brazil with a chance to see snow and some of the absolute best artisanal chocolate in the world!

Hell when I was a kid I saw snow at ocean level even!