Thoughout the years I've used two different paint solutions for my Sergent couplers; Rustoleum Rusty Metal Primer in the rattle can and Tamiya Red Brown in the rattle can. I like the results of both colors. The Rustoleum is definitely more rusty red than the Tamiya, so it's a bit of personal preference. I've learned (from this group, I believe) that the Tamiya paint in the rattle can has a much finer pigment than the Rustoleum, so one gets a bit more of a smooth finish with the Tamiya. As small as the couplers in HO scale, I can't tell much of a difference. The great thing about the Tamiya paint is it's available in both the rattle can and a jar. That allows you to have a jar of the paint on hand to easily touch up the couplers if the paint wears off from operations.
When I paint the couplers, I do it before assembly. I've constructed a jig after reading an article by Pelle Soeborg wrote in Model Railroader a few years ago. The jig is very simple; six small clothes pins glued onto a paint mixing stick. It works great for painting batches of couplers. I take the knuckles and place them on a piece of painters tape taped onto card stock. The couplers are sprayed only on the tops and the sides in my spray booth. The knuckles are painted on the outside (the side we see) only. I then assemble them in the appropriate jig and complete the process. I don't paint the underside covers of the couplers, although there's nothing preventing me from doing so. The underside is not seen, so I don't bother wasting the paint. I have not painted the couplers after assembly because I fear the paint will gum up the hinges around the knuckle, and impede operation.
Hope this is helpful.