I just paint them.
I keep an inventory of Kadee 158 couplers in three colors - black, boxcar red and rust. The painting is done by placing x number of couplers on one edge of a strip of masking tape, bottom of the shank firmly pressed against the tape. Then I fold the tape lengthwise over the whisker side, pressing lightly. I have not done a destructive test as to whether that whisker could be pulled out of its mount, so I don't take chances and only lightly press the tape against that side. Then I take them out to the paint stand.
Sometimes I'll drop a pair of couplers of the appropriate color into a kit box when I'm first figuring out what paint and lettering the car should get. Even with B&W photos of the prototype, if you have a good end view you can often tell what color the couplers should be. But as a default, older cars (from my perspective of 1960-62) often have the couplers painted the same color as the car body. Cars that have their original builder paint scheme, by that era, often were unpainted (rust color). The mandate to leave couplers unpainted so as to allow examination for defects came later.
In recent years, I have painted most underframes black if that's the appropriate color. I'll mount an assembled Kadee 262 coupler box before painting, then usually install the couplers after the paint has cured. The car body may or may not have been finished by that time, but I've often painted the body separately if it's a house car. Gons and flats where there is a concealed weight obviously get a different technique. Bottom line is I don't treat the couplers delicately with regard to painting, and no harm seems to come.
I might also paint the couplers along with the car, if the underframe and body are all the same color.
In all cases, I use the Kadee coupler pliers and bend the pin upward slightly, and check to be sure the spring is in place, before painting. I do apply just a light coat of paint. I rarely if ever find couplers frozen by the paint. I did occasionally find that when I did brush painting of couplers. On rare occasions I might need to apply a brush coat of paint, say on cars I picked up completely built by someone else, and in that case I'll gingerly do a little highlighting on the coupler body and knuckle top and sides.