The extension finish most likely to match is the standard black finish.
The parkerized finish is a dark gray color.
This is usually advertised as "blue" but it's really a satin black color, usually a sulfate finish, which Remington uses on many of their shotgun small parts, like the shell lifter.
Although you can cut barrels down, I'm assuming your barrel has a rib, and this makes cutting a problem, since you have to cut on a web section.
Rib removal is a major pain, plus requires refinishing the barrel.
I recommend buying a new barrel and keeping the original for sporting use, or resale.
If you decide to buy a Choate extension, here's the best deals I ever found:
Eldon King:
gunrunner@cox-internet.com
I suspect he has an "in" at the Choate factory, since he has all Choate items at a better price than any other I ever found.
He has all Choate extensions in blue or parkerized, in any length, and for other guns besides the Remington.