I made a firefly texture default replacement changing the color from turquoise to a warm yellow. The problem is this: when sims catch fireflies, the light is still a turquoise color. I tried editing the colors in the txmts from SimNopke's animation fix, but that didn't change anything. Does someone know? Is it even possible? I think I've read somewhere that changing lamps' lights is editing a .nlo file or something, is that what I would have to do? If so, where do I find this resource? Many thanks in advance!

In my experience, editing the .nlo is usually the way to go. The files can be found under '[your install directory]\Electronic Arts\[relevant EP]\TSData\Res\Lights.' However, I couldn't find anything firefly related there. This likely means that it's instead using a '_lght' ressource instead.

Cloning the jar in SimPE, I see that it indeed has a ressource named 'jarFireFlies_jarfireflies_light_lght'. You should try editing that.

This thread explains the process of editing the relevant ressources.

Thank you so much! :-D I have to try this when I figure out why SimPE isn't my friend today

Update: I had a Windows update waiting to be installed and after that SimPE worked like normal. I didn't get the "jarFireFlies_jarfireflies_light_lght" resource when cloning the jar in SimPE, so I extracted it myself manually. Skimmed through the tutorial you linked and figured out how to edit the color:

these three are the things you want to edit to change the color of the light, for anyone else wondering :-)