Hi Tim,
The color of prototype couplers depends on a variety of things such as the era of the car, the time since the car was built/refurbished/repainted/repaired, the prevailing weather in the area the car operated, the commodities typically carried and the amount of use the car saw.  In earlier eras, painters could overspray couplers when they painted cars, but later (and I don't know the date) that was prohibited along with painting car wheels.  The change had to do with crack detection in the castings.
I find a good source for U.S. prototype photos are the Morning Sun Color Guide books.  The photos are usually clear and well lit by sunlight, and the color reproduction is generally superior, except for the earliest books.  I often use my small collection of Color Guides to check various aspects of freight car weathering, including couplers.
All that being said, often, couplers on 1930s-1950s U.S. freight cars are just rusty, and weather to a hue that is close to the red-brown of most U.S. freight cars.  On a weathered car, it can be hard to tell whether the coupler was painted or not.  In most cases, they would not be painted.  If you move up to the 1970s, '80s and beyond, other factors come into play, such as commodities that have spilled over the exterior of the car and colored nearly everything.  Kaolin covered hoppers are a good example of this - even the couplers are mostly white.  Also, freight cars of this later era were not affected by soot and cinders from steam locomotives and industries which darkened and corroded everything.
Going back to the 1950s, which I model, I often see that the inner faces of couplers and the outer faces of the knuckles have  a more orange-ish hue due to routine chafing of the parts against one another as the cars are hauled about and switched.  As you probably know, newer rust is more orange than older rust which moves progressively to reddish, then brownish, then nearly black colors.  Often, couplers on little used maintenance equipment are nearly black or a slightly blueish black from lack of use.
I hope this helps.
BTW, you didn't state the scale, era or geographic area of your modeling.  Letting us know that might generate some more specific responses.