When the Pioneer Insignia was authorized, in '58 I think, Infantry's branch color was white not Infantry Blue (the color change to blue later that year). It is logical to assume that Insignia that was issued to troopers in the Infantry Regiments at the beginning of the war was white, because that was on hand. Knowing John P. personally and have spent allot of time looking at his extensive collection of chevrons. The original Civil War Pioneer Chevron That he has is white. The Insignia that I use on my union sack coat is white. I think that both colors are correct, but white is more authentic.
My guess on why only white chevrons service is in the nature of the pioneer's job. In combat, pioneer's would be used to create and exploit a breach in the enemy's fortifications. In modern times a Sapper unit in the breach is expected to take heavy casualties. This would be no different for the breach force in the Civil war. Now you have to remember that the Corps of Engineer Troopers of the civil war were being used for bridging, fortification construction, road construction, and sedge work. Pioneers from an Infantry Regiment (2 per Company) routinely would be detached and assigned to the engineers as a work force.
Hope this helps. If you want some help on your pioneer impression just let me know.
Brian N. Clason
Pvt. 26th North Carolina
US Army Corps Of Engineers