
Anyway, Hellser was talking about custom colormaps and I was reminded I had this program laying around. I went ahead and modernized it a little bit and then ported it to Windows. The original DOS version is available as well.
To compile the source on DOS, you only need VBDOS - no other dependencies are required.
To compile in Windows/Linux/Mac, you need QB64. For Mac you will also need XCode (which QB64 depends on). (Use the source from the Windows version to compile on other platforms)
Provided are binaries for DOS and Windows. If anyone wants to compile for other platforms, feel free.

Some features:
- Customizable "end" level - Doom's original colormap did not actually end at the 32nd level - which is why at the darkest colormap some things are still visible. Hexen/Heretic/Strife fixed this.
- Customizable "fade-to" color - check the included fogger.cfg for details.
License:
- This is a really old program. I don't particularly care how you use it. I would prefer attribution if you distribute your modifications, but I am not overly fussed about it. It's really just a simple algorithm, after all.