The purpose of the Late Roman Draco standard.

[Robert Vermaat]

Not really, if the Draco's purpose denotes exactly where every units commander is, the last thing you want is for the enemy to know exactly where the overall commander is, which would happen if the overall commanders Draco was a different colour ('Head for the Purple Draco men, thats where the Roman Emperor is'). Hence, by having every Draco a uniform colour, purple or a similar colour, makes complete sense as the enemy will have no idea where the Emperor or whoever else is in charge is located within the Roman army, they will only know where every commander is located.

Since you would not want a 'wood of dracones' it would hardly make sense to award every commander with a draco. hence, either the Emperor is dressed as a common soldier (I think not) or he is dressed as a general (more likely), but he does not have a personal standard next to his other commanders. or else, he would have been 'visible' because he would be one of the officers at the spot where several dracones could be seen.

Btw, I don't think that Ammianus' remarks can tell us anything about the draco colour (which could have been uniform in colour, indeed. Or not). Rather, it tells us that the emperors were either not dressed as such, or too far back to become targets. Julian does not seem to have been anywhere in front at Srassbourg (when he was killed in Persia he had not dressed up properly) and Valens may not have had a draconarius with him anymore. It's not clear enough to draw such conclusions upon.