Excellent explanation, Preach. Thanks!

I'm not the person who asked the question in #20892, but this is really valuable knowledge. 

Can you use an engine to edit create .lit without the map source and re-compiling?

I know you can make rt lights in DP but I am looking for a way to make a .lit file from existing bsps and hopefully add some colored lights here and there. 

There was a tool made by MH which would automatically generate colored lighting on existing BSPs from the greyscale lightmaps, by using nearby textures for color hints. It does not allow tweaking or editing AFAIK.

Called "MHColour", it's not easy to find. These seem like it:

https://www.quaddicted.com/files/tools/WinMHColour2010.zip

https://github.com/dsvensson/mhlight 

Whoa. That's pretty wild. Thanks. Will try this out. 

If the light entities are still in the map, could you:

1) extract the entities as plain text into a .ent file
2) add color keys to selected light entities
3) merge the .ent file back into the BSP file
4) run the BSP through a coloured-light aware light.exe tool to generate the .lit

I know there are tools out there that can do steps 1) and 3), but will have to let someone else remember exactly what and where they are. Also worth considering that without knowing the exact light.exe tool and settings used on the original compilation, you may not get exact fidelity when re-lighting the map. Probably can get close with some trial and error I imagine, and in some sense changing the lighting is the objective...

One last worry is that the .lit file may only be compatible with your recompiled map, but not the original BSP file - please don't assume that without testing! 

Good info. Yeah there are quite a few options to extract and "inject" .ent files into BSPs. EntEd, AdQuEdit and Quark to name a few.

Over on Discord Woods made me aware of a commandline option in ericw's tools to extract .lit data from a BSP. I'll be working on this over the weekend so I can report back.

All of this might make it into a video as well.