Why are airplanes white?

Here in Oregon, where there is often white overcast and snow in the
mountains in the winter, there's a lot of white. It seems to me that
painting your aircraft white is akin to camoflage. Wouldn't it be better if
airplanes were missile red, cub yellow, construction orange or some other
extremely-visible color?

Once over Estacada while I was soloing toward my private ticket I saw a
camoflaged ultralight puttering along near the airfield over a forest. One
has to wonder: what completely ignorant A-HOLE makes his aircraft
deliberately difficult to see, and then flies into airspace regularly used
by student pilots? That's suicidal, but I digress:

White is a poor color for aircraft because it's hard to see except against
blue sky. Is there a particular reason it's so popular?

-c