What is the difference between Diet Coca-Cola in the red can and Zero Coca-Cola in the black can?
Nada. None what so ever.
You can't believe everything you read on the Internet. I drink plenty of both. I had read on the Internet that the Diet Coke was the basis for the new Coke formula, and the Coke Zero is based on classic. I don't know if I would go that far, but they taste different. Have someone pour them in unmarked glasses to confirm.
Note that the igredients order is different between the two products. Of course, the "natural flavorings" are different in each product.