Re: cove molding
There is not rule of thumb that it has to be one or the other or either. Most of the crown I've put in has been semi-gloss white while ceilings are more of a gray white and wall colors varied. One install job was a yellow bathroom with Hunter Green for the crown (hunting motif ). Wood walls tend to get matching wood moldings.
So, to answer your question, the crown can be painted similar to the ceiling for a more elegant look, or it can be painted a contrasting color to set it off and become part of the room decor. Other than wood walls, I don't know that I'd match wall colors because crown is a means to separate the ceiling from the wall and give it a bit of a picture frame effect. Blending it into the wall it would become lost. The caveat to that is is the walls are papered with a pattern or various colors, then pick up one of the more subtle colors for the crown and the walls will really pop!
I suffer from CDO ... Its like OCD, but in alphabetical order, LIKE IT SHOULD BE!!!