From Tech Insider:
Every airline has their own branding on their planes, but for the most part, they're all painted white.
Why? The light color reflects sunlight and solar radiation while the plane is in flight and keeps the cabin cool. "It's basically like putting sunblock on," MIT Aeronautics and Astronautics professor R. John Hansman told Business Insider.
Read more: http://www.businessinsider.com/sai