Late Roman Army during the 5th century

[Flavivs Aetivs]

It's debatable actually. The Roman Army of the west likely numbered (estimate of course) around 90,000 men total in 395. The Notitia Dignitatum was written roughly in 420 in the west, and with a firm hold on Africa the Army likely numbered 60,000 under Constantius III and Aetius (prior to 439). Afterwards Aetius probablt had about 30000 men spread throughout the empire, which declined further after the Illyrian and Gallic portions left after the death of Majoran.

The Roman army would still have been distinguishable by Shield Patterns and Equipment, but would have looked similar to thier foes. However; most Roman Troops (in the front ranks at least) wore metal armor, so that would have been a big way to distinguish.

The Army had never been entirely "Romans." After the 230s everyone in the empire was a "Roman" or was at least Heavily "Romanized" despite thier ethnicity. The Army had Celts, Romans, Germans, Egyptians, Syrians, Spaniards, and Greeks in it prior to the Barbarian Invasion.

The Army was a Roman army, the difference between sides would be thier commanders or who paid them. Aetius had Burgundians, ex-Roman Veterans, Huns, and Goths in his retainer under his pay probably. Same would apply to Stilicho, Constantius III, Avitus, Sidonius Apollinaris, and Ricimer.