White vs. Silver
Whether you choose a white or a silver coating, you will save on your energy bills simply because of the metal itself. Metal roofing is naturally more energy efficient because it reflects the majority of the sun's radiation, while asphalt shingles absorb the majority of the sun's radiation, locking in the heat. The color with which you coat your metal roof will determine the extent to which you can reflect radiation because some colors are more reflective than others.
White Coating
When it comes to metal roof coatings, white is your best option. In fact, in 2009, U.S. Secretary of Energy Steven Chu encouraged Americans to paint their roofs white as a means of combating global warming because of the increased energy efficiency. In other words, even asphalt shingles can benefit greatly from the reflective benefits of a white coating, but white metal roofs have far greater protection still.
Why it Works
Colors themselves appear as the result of light reflection. In other words, a leaf appears green because it absorbs all of the color frequencies except for the frequency that the eye recognizes as green. White is the only color that contains a mixture of all visible light wavelengths, and therefore reflects all of them and absorbs none of them. For this reason, light is the most reflective color.
Other Consideration
While a white roof coating can provide optimal protection for a home, you may consider other factors when deciding between a white and a silver coating. For instance, depending on the look of the house or building, a silver coating might provide more aesthetic appeal, and in colder regions, heat reflection may prove less of a concern. In such instances, use your own discretion to decide which option is best for you.