Here's a difficult question to answer without getting involved in a heated internet argument that eventually spills over into real-world violence -- why are big planes mostly white? Shouldn't Branson's Virgin fleet be red? Is there some sort of law? Does it help with visibility?
There are many reasons for it, all quite small and subtle. First up, and seemingly the most critical reason for a white coat, is visibility. Not being able to see it in the sky, but visibility of corrosion, cracks, leaking oil and suchlike on the ground. White shows up this sort of thing best, so from a safety standpoint, white is a no-brainer.
Also, as anyone who did GCSE physics ought to know, white reflects heat, so a brighter outside coating helps keep the plane cooler. As a popular answer over on Stackexchange says: "...some airframes require the use of white paint on upper surfaces to keep some elements within limits."
There's also the issue of psychology in place. White paint doesn't fade. Who'd want to fly on a plane that had a faded and weathered exterior? You'd pick up your rucksack and start planning a nightmare coach trip instead, upon seeing a faded plane waiting on the tarmac. White shows up dirt more, but it's easier to jet-wash a plane than to completely repaint it. I'd imagine.
Other reasoning behind white aerial fleets is that white paint is generally cheaper and more readily available, slightly lighter, and that it's easier for companies to apply their logos to plain backgrounds without worrying so much about design issues and changing fashions in colour. So that's why. [Aviation]