In the Army of the Potomac the ambulance corps, after it was officially established used green chevrons. NCO's had inverted green chevrons (point up) while privates had the plain green half chevron. In the Army of Northern VA prior to 1862 members of the ambulance corps had a card they carried that denoted who they were. After 1862 ANV general orders specified the the red band with ambulance corps written on a white background. The orders specified these bands were to be tied around the hat so it would be visible on the field.
your obedient servant,
Surgeon, 2nd Brigade
"not really a surgeon, but I did stay at a Holiday Inn Express"