Regardless of the date of production the finish (color and texture) of the slide and receiver should be very close, at least on near new pistols. Other parts such as the safety, hammer, slide stop, barrel, Barrel bushing, extractor, grip safety, etc. were noticeably different in color, at least through mid 1917.  Between serial #2400 and about serial #160000 the small parts had a smoky blue finish that appeared lighter than the color of the slide and receiver.  The small parts were generally harder than the slide and receiver, and their color was always a bit lighter as well as having a smoky appearance.  

After mid 1917, the color of the finish darkened a bit on the slides and receivers and it darkened a lot on the small parts.  The result is that near new late 1917 pistols appear to have all of the parts about the same color.  However as the pistol received more wear, the slide and receiver appeared to get lighter faster than the small parts which were harder and were somewhat more abrasion resistant. Many pistols that have had a lot uf use show very little finish on the slide and receiver but the small parts retain a lot of finish.  Many small parts where supplied from sub-contractors or where blued in different batches, especially in wartime, and this may explain some random variation between small parts.  

Parkerized/blued bullet feed ramp finish:
Bullet feed ramps where blued in receivers to serial number 710,000, then at SN 710,001 they where machined after the receiver was blued or parkerized, leaving this area of the gun in the white and unfinished. This area should show machine marks and not be polished. In mid-1944 Colts bullet feed ramps where parkerized, this began at approximately SN 1700000.

Types of Finishes found on 1911/1911A1s:
(Serial number ranges are approximant) 
Colt 1 to 2400:
Bright furnace blued, Slide and receiver highly polished. Certain small parts have a high polished bright Fire blued (Cobalt Color) appearance. "Oil Finish" Yet some seemingly original pistols within the serial range have the later duller finish.  These pistols might have been held back to perform some remedial rework and by the time the rework was completed, the acceptable finish had changed to the duller finish and they then got the later finish.
Colt serial #2401 to about serial #160,000: Fine brushed looking Finish, furnace blued.  Small parts have a smoky blue finish that appears lighter than the color of the slide and receiver.                                         
Colt serial #160,000 to about serial #400,000:
Colt brushed blue finish, Coarser girt size on close inspection. small parts have a blued finish that appears to match the slide and frame on near new pistols.
Colt serial #400,000 to end of WWI: Very coarse blued finish causes pistols to almost appear black. These pistols are referred to as the "Black Colt" or "Black Armys". This was due to changes implemented to speed production and reduce cost.  Among those was the elimination of the fine polishing step.
Colt Post WWI to 1941: Colt brushed blue finish, Coarser girt size on close inspection. small parts have a blued finish that closely match the slide and frame.
Colt 729,993 to780,000: First parkerized pistol.  Parkerizing adopted as the standard finish.  Pistols in this range are observed to have a mixed assortment of  blued and parkerized small parts..
Colt 780,000 to end of war production: All pistols and small parts are parkerized.
Ithaca 856,405 to about 900,000:
Du-Lite bluing over a sand blasted surface. Small parts are blue or sandblasted blued.
Ithaca 900,000 to end of production:
Parkerized finish, some blued small parts where observed in these parkerized pistols as well.
Singer S800001 to S800500:
Bluing over moderate polishing, resulting in a light colored non reflective finish.  All small parts appear to have the same color, texture and reflectivity.
Remington Rand 916,405 to 995,000: Du-Lite bluing over a very fine sand blasted surface. The entire pistol appears to be flat blue gray in color. Small parts are blue or sandblasted blued.
Remington Rand 995,000 end of production: Parkerized, also some sand-blasted blued are intermixed during the early production (1,015,000). All small parts parkerized.
Remington UMC: Very similar to Colts Brushed Blue Finish, however the finish was neither as well applied or was as durable.  UMC pistols are often seen with areas of finish flaked off.  UMC pistols are also often seen with little finish and no significant wear to the metal, as if the finish dissolved. Coarse grit on final polish. small parts have a blued finish that closely match the slide and frame.  The degree of polishing and the quality of the finish seems to vary considerably between pistols.
Springfield: Chemically blued finish using the browning process, color was deep blue-black result. small parts have a blued finish that closely match the slide and frame.
Union Switch & Signal: Du-Lite bluing over a sand blasted surface. The pistol appears a dark flat blue grey in color. Small parts are blue or blued over sandblasting.
Reference Charles Clawsons “Colt .45 Service pistols”.

Click on the image and notice that the slide stop has a slight color variation from the frame and receiver. This is normal.

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