I'm trying to recolour the glass in Numenor's lantern here. https://modthesims.info/d/55958/tes...or-lantern.html

I've got to the point where the colour is fine when unlit, but it goes back to white when lit. I have (or thought I had) changed the TXMT (StdMatDiffCoef) colour for both the lit and unlit states.

I tried to see what he'd done in the green glass recolour here. https://modthesims.info/d/56065/rec...ntern-quot.html

But even that goes back to white when lit.

So I poked around further and found this. https://modthesims.info/showthread.php?t=103817

Which says that to make a custom light you need to add it to the Lighting file. But there were no instructions for doing that in Numenor's post. It also says that you need to add it to the CRES and I couldn't find any evidence of that either.

Does anyone know what I might be getting confused over/doing wrong?

You don't need a custom light for this. You only need a custom nlo if you don't want to use an already existing maxis one. Which tbh, is rare to actually need one given how varied the lighting definintions options are in game.

Not sure what is up with your game, but that green recolor is fine. I'm guessing you think it's white because it's a very very light green when lit. I think what you want to edit for the non blinding white is the stdMatEmissiveCoef.