Color is everywhere in The Night Circus. The story narrates in beautiful prose about reds and greens and a variety of other colors, but the two dominating colors throughout the story are black and white. At times they represent the simple division of night and day. The circus only operates at night. It's open to the public, when time is in the black (night), but the circus has a way of enlightening people that attend it. That is never more true than with the character of Bailey. He's obsessed with the circus, and through Poppet and Widget, he eventually becomes the circus's savior.
Black and white are also symbolic of time in the story. Poppet and Widget serve as concrete symbols of that color/time union. Poppet has the ability to see into the future, and she is always wearing white from head to toe. Widget, on the other hand, has the ability to "read" people's past. He always wears black.
Gray is equally as important as black and white within the story, because gray is both black and white together. Or, it's neither. The character of Alexander is essentially an immortal. His past is long, and his future is going to be equally long. He serves as a balance to the past/future dynamic, which is why he never appears wearing black or white. He always wears gray.