Back in the steam powered days (very recently for Delta IT) our schedules were all done on paper. The color of the paper made it easier for crew scheduling to sort them after we dropped them in the box. White=line holder asking to pick up a trip. Green=overtime request. Yellow=reserve (high or low) asking to be first or last to be called out. Sometimes it behooves you to go high for a good trip to avoid a bad trip, or to fill up.