A much more powerful trick for the white foil is the ability to put a full-color image, even a photograph, on any colored paper maintaining full color vibrancy.
Here's the trick for printing a color image on ANY colored paper. (Reference these two dragon images.) A silhouette image is first created from the full color image, then reduced by 1pt size. White foil is then added on top of the black image. Now just run the 'white foiled' image through the printer again to lay down the color on top of the white foil. We previously reduced the silhouette a bit smaller to protect from any mis-registration when run through the printer a second time so there is no white creeping out from any edge.