Amid the sea of red, white and blue adorning shirts and flags worn by soccer fans on Governors Island Tuesday as the U.S. Men's National Team battled Belgium in the World Cup, another color could be seen dotting the audience, if one looked closely.

On a hat here, and a shirt there, it was the sky blue of a team that doesn't even exist yet: New York City Football Club, the built-from-scratch Major League Soccer franchise that is set to start playing at Yankee Stadium next March. Before it takes the field, NYCFC is facing its...