At Wizards of the Coast one of my main projects was working on Magic the Gathering, a tabletop and digital collectable card game.  Starting from a photograph of the original artwork I would color correct the image, making sure the digital version would maintain the original colors as closely as possible for cmyk print.  When the card was to include a foil version, I would then create an  underprint version.  I would decide which feature of the card should show the foil treatment and how much of the foil should be visible.  To create this effect we made a grayscale version of the artwork, blocking out parts of the image in black or darker shades of grey to block the foil from showing and the white or lighter grey colors would be where the foil would show through on the physical card. To create this effect I would clip out all the different parts and features and then fill them with different levels of black and finish it off with a low opacity layer of the actual art work to help with fine detail.