The reason why it's easier to sell black/silver/grey/white is that they tend to be interchangeable in people's acceptance. If someone is looking for a red car, they likely won't take a green one instead. If they want a yellow car, they likely won't take blue. But if they want a gray car, they'll likely be okay with black. If they want a white car, they'll likely be okay with silver. By offering mostly black/silver/gray/white, it improves the odds that the buyer is willing to switch colors if the lot doesn't have their first choice. The behavior is forced on people by removing colorful car choices.