Here's how this trick works:
Printers spray ink as tiny dots into precise patterns, a standard technique called halftoning. Different patterns of cyan, magenta and yellow dots produce a wide range of colours.
When the halftone is printed along lines onto metallic sheets, the researchers noticed that the resulting colour depends on the viewing angle. This is because incoming light traversing the ink lines cast shadows onto the metallic surface. Ink lines perpendicular to the incoming light create a large shadow and appear as "strong colors." Ink lines parallel to the incoming light do not induce a shadow and appear as "weak colors." When the print is rotated by 90 degrees, strong colors become weak and weak colors become strong.
At the moment, the technique only works if you use an inkjet printer and print on a metallic sheet. But the researchers believe it has a lot of other potential and more useful applications, particularly in the realm of security. The EPFL says it could eventually be used to print security elements for passports, ID cards or paper bills that counterfeiters will have a tough time replicating.