How to create an inventory item with multiple properties

[cdarken]

Say you have to manage an inventory of hats. These can be identified by color and size. How to you create an inventory item and add these properties to it? Should these properties be metrics? But now you can only assign one metric to an item, right?
What would you advise?

[cdarken]

This seems to be a duplicate of #50. I will try to handle it with variants. Sorry, didn't find it because of the not very descriptive title.