When in Prestashop you click in the shopping cart on the name of a product you come in page of that product. So if you have a green dress in your shopping cart (with color being an attribute) and you want to change the color to yellow you need to delete the green dress and add a new – now yellow – dress.
It would be better that when you click a line you will be enabled to edit it. You should just change the color attribute then and press and “Update” button.
This would also fit with other proposals to make the software more attribute composition friendly. Just imagine that you have just composed a computer for yourself and set the right memory, harddisk, mouse, screen, etc. And then you change your mind about some item and you need to enter everything again.