Marco wore a mask to hide his true face to everyone, and in a way some of the others kept their true identity to themselves, including Tsukiko. The mask shows how some people had something they wanted to hide.
The circus opened and night and closed by dawn, i think that the circus chose the night because of the sense if mystery that came with the dark, which is the same feeling the circus brought as you never knew what you would stumble upon next.
White is a color of innocence, in which everyone in the story once was until their involvement in the circus. For instance Bailey was an innocent little boy until he got more involved with Poppet and Widget in the circus.