There are a few reasons... first off, white is a cooler color in the summer time. Darker colors will heat up cabins on parked aircraft a lot faster than lighter colors (and most corporate aircraft sit around much more than the typical airliner).

Second is that darker or more vivid colors tend to stand out more on the ramp. Most private aircraft operators don't want to draw excessive attention to themselves. Thats why you don't see a lot of private aircraft with corporate logos either. For the most part they want to come in and out without drawing attention to their passengers or the reason that they are in town.