colour_overlay.reset(NULL);

COLOUR:  . = green / blue:5 / red / none
         COLOUR: allows you to attach explicit colours to any feature.
         Explicit colours will override the default colour for that
         feature. The example shown above colours all . (floor) in the
         map green, blue, red, or unchanged (use the default colour).

         You can use : to specify that all glyphs get the same colour:
         COLOUR: x : red / blue
         will colour all rock walls in the map red, or all rock
         walls blue.
         COLOUR: should be used very sparingly, and only for features
         where it won't cause confusion (i.e.: never re-colour features
         like lava!)