Dave, the colors actually mean different things.
Green meant another section was following, ie as 2nd section of a scheduled train, running on that train’s schedule but perhaps 20 minutes later. It was not an extra as it was considered a scheduled train and treated as such. Rules permitted multiple sections, and heavy trains were often broken into sections. This was common with passenger runs, and esp during WWII. The last train running on that schedule would have no flags or class lights showing, which told everyone it was the last section. So if you were an inferior train waiting in a siding for a scheduled train ie Timetable listed, and you saw green flags on an opposing train, you knew you could not move until you saw a train moving in the same direction with no flags. And you might see three trains in succession with green flags. But you could not move until you saw the last section of that train passed you.
White means extra, as in a train that has no schedule and is inferior to all other trains, except per direction. An extra moving in the superior direction would have rights over an extra moving in the inferior direction. If you were flying white flags you had to get out of the way of everything else on the road.
Doug Harding
www.iowacentralrr.org