White socks are the default choice for road riders, although a few riders have used black and other colours.
Why? Well in the late 1920s Charles Pélissier decided his legs were so tanned and impressive that he felt a pair of white socks would help him show off. Until then black and brown, yes, were normal, understandable given that most roads were dusty and dirty.
Pélissier was probably the first pro rider to exploit his image. Not content with merely winning races, he made a name for himself using the media to relay images of style and poise. He was often beaten but at the time he was the highest paid rider in the sport during the economic boom of the 1920s.