In Westeros, bastards who take up their house banners must reverse the colors of the sigil. This custom is meant to signal the man's bastard status. House Stark's sigil is a gray direwolf on a white background, which means Jon Snow's banners will show a white wolf on a gray background.

Jon Snow, the King in the North, will be known as the White Wolf. The fact that he owns a snowy direwolf is really just a foreshadowed cherry on top.