The order typically runs with not just milk but all the supplies. It's just the "going out of milk" trigger happens most frequently.

And yes, using different kinds/colors of cards for different kinds of milk might have been an improvement to consider. It would, however, make the system more complicated. Was it me, I'd start with a simpler design and iterate only when I saw the need.

It follows the rule of thumb: make it as simple as possible.