Answer: Cold Foil!
On a 6-color (or more) press, the first 2 units are used for the foiling.
In the 1st unit an adhesive is “printed” on the sheet reflecting your foil artwork; think of it like a spot varnish. The 2nd holds the cold foil roll. As the paper moves through the press, the foil unrolls, only adhering to the pre-glued areas.
Theoretically your foil options are many and even include a holographic option. But the most used foil colors are Silver, Silver, Silver and, very occasionally, Gold.
Why Silver, you ask? Silver cold foil is a “neutral” choice. And with the CMYK stations on your offset press, you can print patterns, images or text in any CMYK combination. Thus you can create any foil color in the rainbow, even a Gold effect! (PaperSpecs PRO members, don’t forget to get all the crucial information you need on a wide variety of foiling techniques in your PRO Guide to Foil!)