Painting knuckle couplers

In this photo a commenter said it is illegal to paint knuckle couplers. This sounds true since the paint would gunk up the mechanisms, but I wanted to hear your opinions.
http://www.railpictures.net/viewphoto.php?id=130441

Wheels and Knuckels are not allowed to be painted, since their cast paint can hide cracks. only place you see painted wheels or tires is overseas.

Thanks DutchRailnut for the heads up!
Star Trek 40th Anniversary