On roofs with adequate insulation, heat transfer through the building is not often an energy efficiency problem. Under-insulated structures, or roofs subject to extended heat waves can benefit from reflective coatings in that moderation of surface temperatures will improve the efficiency of the existing insulation.
For the most part, energy efficiency is gained via white or highly reflective coatings when HVAC intakes are mounted on the roof. Black membranes like EPDM, modified bitumen or highly conductive metal panels, will absorb and retain substantial amounts of heat. This results in high ambient air temperatures surrounding the air conditioning intakes, which in turn requires AC units to use more energy to cool the super heated air. With a reflective rubber roof coating, the surface temperatures are moderated and the AC will not have to work as hard to deliver the desired cooling benefit. For a better sense of the cost benefit that can be realized when converting dark or conductive roof surfaces, to highly reflective rubber surfaces, the Department of Energy provides a basic roof calculator as a guideline.
For additional details, the following link may be of interest: http://web.ornl.gov/sci/roofs+walls/facts/CoolCalcEnergy.htm