Well Gentlemen,
i'm bit surprised by your consensus of "lack of sources" on roman military clothing when we have two main ones : mosaic and manuscripts.
I'll deal only with late roma era i know best than the previous ones : on that timeframe we do know that military clothing was very diverse, in its raw material, quality, color etc etc.
Basically the source of military clothing was the local production made
by civilians for their own purposes, but part of it was kept by the nearest unit under the regime of the annonae, a tax, once paid with grain, then with money, and with the slow collapse of roman economy with anthything-with-interest-for-the-army, and clothes was without doubt a very important one.
They were even fabricae (factories) specialised in this production, but with more than 300 000 soldiers to supply, surely insufficient.
So obviously a legio based in Egypt (linen ?) and an another one in Britain (wool ?) certainly not weared the same clothes. And i'm pretty sure than even within Egypt the different roman units carryed different clothes, and naturally different colors.
As we well know in our modern world, clothes (and fashion) is a status symbol, showing the importance of the people wearing it. Off course it was the same in the roman army. Officers were recognisable from their very expensive clothes. We know with roman legislation (under diocletian) that three type of officers's tunic costed 1000, 1250 et 1500 drachma.
And we know from papyri that the cost of a simple soldier tunic was... 25 drachma in Cappadocia in 138 (so earlier, and we know that a strong inflation occured, but still..), same price paied in Massada in 81.
But why this huge difference of price : the quality was surely finest, but this is antiquity : there was not big difference between linen, or wools. The price difference comes frome hours of work (hundreds ?) on decoration, dyeing, etc...
Remember that in this era, beautifull clothes was a coveted booty, and the most beautifull clothes were even sended tho the gods during the pagan times...
Back to the sources :
the written sources deals with with withe clothes, but mainly because the all come from egypt. In a payrus, the army ordered "That the articles are made of fine wool, soft, pure,
white, spotless, well woven, tight, well hemmed, pleasant, without defects, of a value not less than the price paid in advance for them."
The vergilius romanus, a fifth century manuscript (amybe from Britain, without certainty), show very detailed soldiers, and they are wearing dark clothes
under their lorica hamata.
then we have many mosaics showing soldiers, and they are wearing very coloured clothes.
i won't post hundreds of pics, just ask google, but here are some examples :