The use of color on the battlefield is well documented. The highly saturated artificial dyes as depicted in some of the images in this thread simply did not exist during the dark and middle ages. The colors should be a lot less saturated to be even remotely believable (NOT like the highly-saturated primary blue color in the previous post), but there SHOULD be a lot of colorful clothing, emblems, tabards, banners, and other items. The majority of the clothing shown so far looks too dark and depressing; there should be more natural off-white cloth (linen, wool, or cotton), a lot of it dyed in somewhat muted or slightly pastel shades.
The official faction color should be used by a lot of troops of the better-organized factions, particularly by their higher-paid professional troops, with far less uniformity among the loosely organized tribal factions or the conscripted levies.
As pointed out, Rome reserved the color purple for its ruling elite. The senators were allowed a narrow strip of purple on their togas, the emperor was permitted unlimited use of the color. True blue dyes were also rather hard to come by in the past (before the 1700s), so you'd rarely see it used for military uniforms where it would require significant quantities of blue dye, although it became fairly common by the 1800s.