In Brittany, there is a type of milk that comes in a bottle with a green top (sometimes a gold top).
This is called Lait Ribot which is a traditional fermented milk that has been made in Brittany for thousands of years (and tastes as if it has been hanging around for about as long!)
It is in fact fermented babeurre, or bas-beurre (literally "low-butter"), which is the white liquid that remains after you've beaten cream to make butter. It is drunk as a snack, or to accompany a meal of potatoes or buckwheat galettes.
Under no circumstances should you buy this to put on your cornflakes - it will make them taste like something that the cat has peed in (not that I'm aware that I've tasted anything that a cat has peed in but, with my 2 beasties - one never knows!)
The manager of my local Epicerie (corner shop) used to take enormous delight in NOT warning English tourists who came to his counter with a bottle of Lait Ribot in their basket! I have the horrible feeling that I bear some of the responsibility for that!