The introduction of single-ply roofing material also brought the opportunity to use roofing material in colors other than black.  The advantages of a light-colored roof over a dark roof, as lobbied by environmentalists and some manufacturers, is high roof reflectivity.  Roof reflectivity is the ability of a material to reflect solar energy, which can reduce cooling costs and energy usage.  

That sounds like a win-win situation, but based on recent studies by the MRCA (Midwest Roofing Contractors Association), the reflectivity ratings (and energy savings) can vary drastically based on various factors.

  1. Location - Roofs in warmer climates will see greater energy savings using a white roof than those in more moderate climates, like Central Illinois.  The concept behind this is simple; The energy savings are realized when it's hot outside and the building's air-conditioner can run less frequently to keep the interior at a comfortable temperature.  Hotter environment = more air-conditioning usage = higher energy costs.
  2. Pollutants - The primary function of the building being protected by the roof is a huge factor in determining the roof reflectivity rating.  For example, if the building is a manufacturing facility that has exhaust vents ejecting dust or cheminals, these pollutants can discolor the white roofing membrane, reducing the reflectivity rating (and attractiveness).
  3. Organic Matter - Biological growth, vegetation, soil and dirt can all substantially reduce the roof reflectivity.  Depending upon where the building is located, this type of debris is inevitable.  Also keep in mind your current black roof likely has the same type of organic growth, but due to the dark color, it's presence is not as noticeable as it will be on a white roof.

To read more about the reality of roof reflectivity, below is an article published by the MRCA.  

The Real Dirt on Reflective Rooftops