You can color passages based on their distance from the entrance. This is useful estimating which areas of the cave are the most remote. This value is not the straight-line distance, but the distance required to walk through all the surveys to reach the station. The statistic is based on the route that was first used to get to the station. It ignores any more recently surveyed short cuts. The colors are arranged in "rainbow" order with purple being the nearest and red being the furthest.
Activating. You can turn on the option by selecting the "Display->Color By->Color By Entrance Distance" option from the menu bar.