It seems that, for the most part, pre-modern armies did not have any uniforms/common attire for various reasons: namely citizen-soldiers who had to supply their own equipment and use of standards/flags to identify units.
So the default assumption seems to be that it wasn't until the late medieval period that you might see an army arrayed for battle with any uniformity to their appearance. The two types of uniformity I'm imagining are equipment and actual attire (uniforms).
For tribal warrior-based armies, it seems kind of obvious that each soldier would have his own equipment/attire. But for more organized armies, it seems like the required discipline and cohesion would lend themselves to perhaps seeing the benefit of enforcing a common appearance.
Is there any indication that earlier armies (most obviously phalanx-based Hellenistic armies and post-Marian legions) made an effort to affect a uniform outward appearance?
It seems like logical arguments can be made either way: you wore/used the best stuff you could get your hands on and nobody cared how you looked vs. discipline and solidarity required a sense of equality/structure that was reinforced by using/wearing similar equipment.
There is the Notitia Dignitatum, this details the shield designs of various parts of the Roman army around the start of the 5th century.
There would have been some uniformity to the equipment of armies that provided the equipment for their soldiers, including the Romans. I would imagine that a Roman Legion would have had soldier that looked very similar especially during the height of the empire.
This may have, to some extent, even gone back to the Persians who may have had some uniformity in equipment provided by the state to some of their levy units.
There were different units in ancient armies that would have had similar appearances even if the army itself was not uniformed, like the Silver Shields in Alexander, and his successors, armies.