You can paint sights just like any other metal. First, make sure the surface is completely free of all oil or solvents. Use (the right) alcohol, or other degreaser.
you may want to use a primer, and I would also recommend some masking tape.
I have used model paint (enamel, acrylic, etc.) with good results. Like any painted surface that gets moved, it won't be permanent (holster wear, bumped, etc.) but it will last a long time, usually. And its easy to reapply.
You might also consider a clear overcoat. Flat/matte would be the best, I think.
I have also used the Sharpie permanent marker (black) to blacken the sights of stainless pistols. Doesn't last forever, but works tolerably well, and reapplies in just a few seconds.
Nail polish also works, pretty well if you do your surface prep right. I don't use nail polish, because my wife doesn't wear any, and I just never seem to get to the beauty counter when I'm shopping......
