Put this material on an exit from a dark room to the outside world.
darken location build a door to the yard material the door to _exitoutside_
When open, the exit is set to find: [the material called sky], which means it will light up the room during the day. The exit also takes on the colour of the sky.
When closed, the exit removes _sky_ and _light_ (just in case it was day when last opened). It also changes colour to brown.
You can also put this material on other objects besides exits.
darken location create a window material the window to _exitoutside_ open the window
Note, even though you can't go through the window, it still changes to the sky colour when open (lights the room as long as there is daylight).
Customizing If you want it to be a different colour than brown while closed, then script the exit with the following (substituting 'blue' of course)...
##getcolour: $colour = 'blue';
If you want different open and close messages, script something like this:
##getopentext: $opentext = 'Toxic fumes begin to enter [$loc] from [$actor]";