I'd like to throw my hat in on this discussion.
I used to work at an automotive pcb manufacturer. (http://circuitboards.com/
) Working in the shear room.
They would print in any color that the customer wanted, at least when I worked there, 5-6 years ago. It was no different in price, but most places just chose the stock green color. It was just a matter of changing the mylar layer on the board. Aside from that, the printing was just silk screening which I saw colors ranging from bright pink, to dark brown.
The only board that I remember having to remain a certain color were the boards that were being baked, which had to be done, before they were even sheered, so I would assume it still would have ended up being the same color process.
Btw, the white on that saphirre board on the last page, is only a layer to prevent shorting things easier, and I believe its the same for any mobo man. process.
edit- It appears that they allow these colors after further investigating the site. Though I remember seeing a much wider array when I worked there. "Taiyo L.P.I. green, black, red, blue, yellow, white"Edited by jesusboots - 2/5/12 at 11:12am