Historical miniatures painters know that it is often very difficult to be sure of historical authenticity in our paint jobs. this is especially true in the ancient period, and doubly so when the subjects of painting are not your run of the mill Greeks and Romans. The fact of the matter is that good source material on the ancient Persians, Indians, and other non-European armies is scarce and conflicting.
My current project is 10mm Persians. I am attempting to "double-dip" and make them playable as either early or late Achaemenids. This is possible primarily due to the fact that I cannot obtain 10mm Persian infantry with spara, or large wicker shields. The crescent shield and round shields were used by some Persian infantry from the time just after Xerxes through the fall of the empire due to Alexander. Maybe I can scratch build some spara walls to stick in front of my troops? That would be a lot of work, though.
However, my main concern is how to paint the outfits on these guys. The Sekunda Osprey on the Achaemenid Persians is not bad, but something like 9/10ths of the plates are of the Immortal (amyrtaka) regiments of the Persian Empire. This means they all wear purple and saffron in varying degrees in the Osprey. It's true, manly fighting men wore purple and saffron. It was the bling of the day, you see.
The Purple People Eaters of Osprey:
However, I also managed to nab the famous Montvert on the Achaeminds off of ebay for a song a while back. Don't hate me. In that volume they show a few more versions of Persian dress. Supposedly (according to Sekunda) the ubiquitous yellow or saffron hood was a sign of Persian ethnicity or Persian citizen status, and the white hoods might have been indicative of non-Persian troops outfitted in the Persian style. The true coloration and significance of Persian outfits at this time, however, remains unknown and probably will for the foreseeable future.
So I ran with that and made my 4 base units of "Persian" infantry as satrapals (warriors serving a Persian administrator who were probably not ethnic Persians) with white hoods, sleeves, and trousers, and a light green tunic/linothorax armor. I gave them red shields, and although they look gaudy, they do so in a reasonably Persian fashion. I'll post some pics soon. In my mind, these come from Anatolia or thereabouts in modern Turkey and would serve an Anatolian satrap like the ones who tried to slay Alexander at the Granicus. I will, however, reserve the saffron and purple for more elite units such as Immortals (early) or apple-bearers (later) and other ethnically Persian troops in the army.
My buddy the Consul is facing a simliar dilemma: what color were Ancient Indian turbans? The sources simply do not say. We know that most line infantry wore plain white garments, but there were multiple dyes available, so turbans could have been different colors. It is not hard to imagine different tribes or other groups wearing the same color for easy identifcation on the battlefield. For the purposes of a wargamer painting units of 10mm figures, such differentiation between units greatly adds to the ease of play by helping us to avoid mixing up stands from different units. However, historically accurate dress might be compromised by such a practice.
In conclusion, I think you've got to be flexible when painting ancients who aren't Roman or Greek, as the source materials just aren't comprehensive. Make the best of your sources but don't be afraid to improvise a bit on the more obscure ancient peoples. Make compromises for ease of identification where possible. We are playing games, after all.
Finally, the truth was probably less narrow than we often imagine. One only needs to study the myriad shades of "German Grey" used in WWII uniforms to see that the exception is the rule in uniform colors throughout much of history.