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....