I share the view exposed that the romans would probably had uniformity on materials (and base colors) but each soldier would decorate his own shield with different patterns, much like in medieval times knights decored his own shields and flags. Probably those designs were not invented or just put there because they were "cool" or something like that, most designs would have a significance, like being the symbol of a noble house or of a god (a devoted man would trust his weapons to the god of war). So my assumptions is that the romans were not different in this era to others peoples, and probably the only uniformity they would have is that their armours/shields and other panoply of war would be of the same material and probably similar colors.