I want to put a label on it, so I did some research:
On the press, four (up to six) different ink rollers supply the colors that will coat the individual printer plates. Printer plates contact a rubberized blanket on a rotating wheel, resulting in a complete negative color image on the blanket. Clean cans are fed into the printer and are placed on a steel mandrel. The spinning mandrel then rotates the can body against the rotating blanket, resulting in the transfer of the final graphic image onto the can body. Other coatings are applied after the ink, and then are sent to the UV ovens. Next, the vacuum belts stabilize and support the cans in an optimal geometry for UV light exposure. The ovens operate at about 110 F and contains between six and eight 10-inch, 300 watt/inch, microwave energized mercury lamps. The lights focus maximum illumination on the exterior surface of the aluminum cans, as well as the interior to insure all ink is cured properly. Finally, internal coating is finally applied and dried through a gas fire oven.
The technical details (above) don't really seem to make it clear how the paint interacts with the can visually, so here is a comparison:
As you can see, the roughness stays about the same from the painted can to the non painted can, but, the painted can is colored. However, its not just changing the color of the metal because the white color apears to be less reflective than the rest of the colors.