Great links and photos. Also thanks to Steve Markowitz for sending me some links.
The colorized photo implies the original color of the columns was brown. However, I have also seen photos of green columns in later years. One thing I've learned in modeling these stations is that there appears to be a precedent for just about anything you can imagine. So I'm leaning towards the same rich dark green that I used for the 42nd Street station module, because I like it more.
I agree that if someone ever models the City Hall Station, that will be a pinnacle in subway modeling! And gap fillers are not required, since the station never had them and only end doors opened on the cars in that station.