Unfortunately, installing a white roof isn’t quite as easy as opening a can of paint. Most roofing material manufacturers (including manufacturers of asphalt shingles) will not extend their warranties to roofs that have been painted or covered in some way. Many low-sloped (i.e. almost flat) roofs are covered with elastomeric coatings that are specifically designed to withstand weathering. These coatings can be painted on, which is why you see pictures of people painting their roofs with rollers.
On a clear day about 80% of sunlight reflected from a horizontal roof will pass into space without warming the atmosphere or returning to Earth.
Energy Star and the Cool Roof Rating Council both provide useful resources for finding highly reflective roofing materials.
Additionally, Lawrence Berkeley National Laboratory has a directory of suppliers of cool roofing materials.
Cool roof programs are available in select locations, see the Cool Roof Rating Council for a list of rebate and loan programs: