Based on some work I have seen, I would suggest contacting Cylinder and Slide (
www.cylinder-slide.com) and see what they say. I don't think the work will be cheap, though.
If you are trying to "restore" the gun, note that 1917 Colts were not originally Parkerized; they were all blued. Early ones had the black appearance of civilian Colts of the era; later ones were brush blued. Only guns rebuilt and refinished in WWII were Parkerized.
Jim