Included for completeness, some unusual Don troops.
The uniform is assumed to be based on the old border regiments, as shown in Shenk and Knotel, which had green facings. White Guard Almanack #8 shows the Don Border Regiments retained this, at least for shoulderboards.
Tsarist border regiments had dress trousers of blue with a green stripe.
Shoulderboards were green with white or yellow numbers.
Buttons and officer lace were gold, so presumably officer's rank in silver.
The Imperial army had had both mounted and foot border regiments. We have assumed the Don units were largely foot, because of the shortage of horses at the time and because they were merged into plastoon units. They almost certainly had a mounted component too, since almost all Don troops did.
The 4th Don Border Division was formed in August 1918 as part of the Young Army. In it were the 1st to 4th Border Regiments. Perhaps they were recruited from non-Cossacks in the Don Host area.
Originally intended to guard the borders (presumably that with the Ukranians/Germans at this point) it was put into the front line in January 1919.
By March it was the 4th Border Brigade, of two regiments. This was placed into the 5th Don Division in May. It was disbanded in August 1919.
There was a Separate Border Battalion in Iudenich's North-Western Army.
There was a Separate Black Sea Border Detachment in the AFSR, which seems to have been in Sochi facing the Georgians.
General Dietrikhs formed a Border Rifle unit in 1922 in the far east.
The current Russian Border Guard has this flag, which is the old Tsarist naval flag combined with the border green!
Quite a few Imperial border regiments had Nikolais (in all green) but we have no record of Don border regiments at the time.
If the Don border regiments followed Tsarist practice, their regimental HQ flag would have been a green square with regiment number inside.
Sotnia flags were presumably green over the usual squadron colours, with the unit number in yellow.