From the looks of it, the guidon looks like a Lone Star from the state of Texas. From the order of battle for Cold Harbor there were only three Texas regiments, the 1st, 4th, and 5th regiments that were at the battle, and according to General Lee he had no regiments in reserve for the battle. There is one account I know of that Barlow's troops managed to overrun a battery of four twelve-pound artillery pieces and captured one flag and managed to bring it back when they were forced to retreat. To be honest, I doubt that this guidon would not count for a color compared to a regimental, but perhaps it was a flank marker for one of the regiments