Yep, Duotone!
A difference in surface texture rather than surface color. Bright finish except for a pleasing soft satin finish within each cylinder flute, the bottom of the trigger guard, and top and rear of of the revolver's frame. Practical for its anti reflective qualities on the frame, wear resistance on the trigger guard, and just because it offered a bit of distinctive contrast in the cylinder flutes. Colt only provided the Duotone finish from 1952 to 1955 then skipped the required steps for Duotone preparation when finishing the revolvers, polishing them uniformly.
Looks more distinctive in person than in my poor photographs.
My trigger guard has a spot of honest holster wear on the trigger guard finish. This 3 5 7 was a working gun for a lawman.
Only the first year of production 3 5 7s had the Colt Accro rear sight with the rounded front sight base and came with the muzzle highly polished and left bright. Subsequent years of 3 5 7 production found the muzzle left blued after the finish process and the rear sight base's front was squared off.
More obscure and esoteric stuff about the Colt 3 5 7 than anyone would ever want to know.
I have this 1953 vintage Colt Official Police revolver on hand. It came with the neato Duotone effect though it is muted due to the revolver's wear. This revolver was a New York Police Department gun. Shown with the typical Jay-Pee issue holster equipment. The Official Police was the renamed Colt Army Special. Both the Army Special and the Official Police were the plebeian work horse Colt revolvers in lawmen's holsters for over 60 years.
NYPD officer's badge number appearing on the revolver's rear grip frame.
The '53 Official Police shown with its World War II bud, the Colt Commando. The Colt Commando was simply the Official Police gone off to war. Sporting a rough finish and molded plastic grips, it was intended as a substitute standard side arm and also to assist with arming guards at American defense plants and other war critical facilities. The government contract requested deletion of the particular polishing steps Colt provided on its commercial production as well as a more durable finish applied. No checkered walnut stocks were wanted, but inexpensive and utilitarian plastic would do. Revolver production down and dirty reflects the rush of the times.
The predecessor to the Colt Official Police, this Colt Army Special dates to 1925 and is less common because it is chambered in .41 Long Colt rather than the ubiquitous .38 Special for which the great majority of these E-Frame Colt models were chambered. The .41 Long Colt is long obsolete, but I handload for the cartridge so shoot the hooey out of this one.
One more old Colt to finish out what Waltesefalcon started here. This big Colt New Service (U.S. contract Model 1909) dates to 1910. Displaying some wear, nonetheless the fabulous Colt blue finish shows through. Nothing touches the Colt products of the late 19th century and first decade of the 20th century for beautiful blue finish. And to think this was purchased for issue to troops with such a fine finish! All but about 300 of the entire Model 1909 contract production went to the Philippines where they were sorely needed at the time. The revolvers all promptly turned brown in the humidity and hard use and that is the way most are found today. This one was likely one of the 300 which stayed stateside and also was not likely to have been issued at all before it was "mustered out" of service in 1920, when a gun club friend's dad bought five of the Model 1909 revolvers and 500 rounds of .45 Colt ammunition from the San Antonio Arsenal in 1920, four to arm nightwatchmen at the ice production, dairy production, and ice cream production plant for whom he was the plant superintendent. The fifth one came home to the Lawson household where my then 13 year old friend appropriated it and most of the 500 rounds of ammunition also purchased from the government facility, for use on the family ranch near Kerrville, Texas northwest of San Antonio and at another family ranch in Mexico, west of Victoria. I'm guessing the wear occurred after the revolver left government stores. Cost of the five revolvers? Five dollars each. Ammunition? One cent per round.

My favorite style of Rampant Colt logo, the prancing pony framed by the stylized "C," used in the first decade of the 20th century. Can you tell it was a partly cloudy November afternoon when the photo was taken? The flawless polishing was applied prior to bluing by some nameless Colt craftsman in Hartford Connecticut for probably 50 cents per day wages.