Yes, they work ! Great !
They're in shades of blue or maybe shades of grey. Brighter at day and darker at night. But it's better than nothing.

It seems the code needs to be updated too when you draw new maps: my underground map crashes on segmentation fault.

And when the "Assert()" functions are on, which is the case when you build JA2 to use the editor, everything you forgot to do like register a loading screen in the code for you new map makes the game exit. Rather annoying when you want to test your map. The underground sectors avoid the loading screen crash though. I think the "Assert(0)" on line 164 of "Build/Loading_Screen.cc" should be removed. If you didn't register a loading screen, you get the default one from the lines that follow the "Assert()". That's a nice behavior.

I'll look at the code to see what I need to do to make my underground map work.