I was thinking of recoloring the main GUI of TSL so that all its elements are in an orange/reddish/brown tone instead of the usual green, but I don't quite know how to do that. What do I need to use to achieve this and how to do it?
To my knowledge, you need to use the KOTORTool by Fred Tetra to extract the GUI picture files. Then you recolor them as you wish, and put them into the override (and/or put them on KOTORFiles so we can enjoy your mod too ;) ).
You can only do so much by extracting the tga files and recoloring them, for the rest of the GUI you need to do some hex editing. I'm not familiar with the process myself but I've spoken to somebody who knows how, talk to Logan23, he's the man that knows.