To help put everything that have been said together here, I'll give you the missing piece to the equation if you haven't discovered it yet. You will need a utility to extract the res files. Fortunately Textures have been done, unfortunately I don't think anyone has made one for the 3D models yet that fully work. That portion is still very much on my long TODO list. The easiest tool out there I know of can be found here: viewtopic.php?f=49&t=57708
He has directions there to build the tool or use the command line version, I know the pu.ush link for his pre-built one is expired. I have a dropbox link of an old copy of it from long ago : https://www.dropbox.com/s/ob265h7b8nqf6 ... rt.7z?dl=0
The process is simple, grab the res file. Put it in the appropriate folder, load the utility and extract the texture as a png. Edit the png, Load it into the utility and remake the res. You then place that res in whatever jar or custom-res folder the client you are using supports.
I do believe it should still work for items like merchant robes despite the tool being a bit dated, but it is pretty simple to use.
If you have any other questions or issues I can lend a hand.
edit: Also its Roman purple not magenta....