I'm using the primary source Notitia Dignitatum as a guide. Each unit has a different & distinct shield design. Some are easy. Some are quite detailed religious iconography. Evidently, the elite units had simpler, even plain patterns so I'm probably not going to have many garrison units (Limitatei) with their detailed & complex patterns.
Here's a sample:

I generally paint a trial shield before doing the entire unit.
The Late Romans will certainly be a colourful bunch.
donald