File naming and placing guide (Assuming someone hasn't messed it up)
http://brawlhacks.wikia.com/wiki/The_Fi ... ming_Guide
Go all out changing the game as you see fit to have fun, but if you go for online matches, you should aim for both sides having the exact same files.
Steps for just a skin replacement:
Step 1: Sort through your files that you want to put in and replace existing files with. Potential files you can have in what you downloaded:
FitLucario.pac (This changes attacks and movements, not needed for skin changes unless the skin is designed for different moves, such as the Goku sets out there)
FitLucario0_.pac (Skin file, needed for changing skins)
FitLucario0_.pcs (Same as above, but packed, best to have both to prevent crashing)
FitLucarioMotionEtc.pac (Handles movement and animations)
Sometimes what you download will be something like Halo.pac or something with a name that doesn't tell what the file is for, you should be able to go off assumptions from what the download says it should be. Skins follow the format of FitCharacter##.pac or .pcs
If you are missing one of the skin files, you can use Brawlbox to open the one you have and export it as a .pair (Which puts both out in the correct formant and fixes potential problems). Brawlbox can also be used to check what the file is intended to be.
Step 2: In your minus folder go to /minusery/pf/fighter/lucario (Or whichever character you are wanting to work with, use the naming guide for things like poro aka jigglypuff).
Step 3: Wherever you have your Mega Mewtwo files, you should have a .pac file and .pcs. Minus recolored his skins some, and I don't remember which went where exacty, but using the guide I'll assume that light blue was changed to the purple that you are speaking of, so you want slot 04. Rename your files to FitLucario04.pac and FitLucario04.pcs. Place them in the lucario folder mentioned in step 2, replace existing files (or move existing ones out as a backup).
Assuming you named the placed the files correctly, your skin replacement should be working. The pack you mentioned has all of the correct files for skins, you just need to pick which color in it you want, and name it correctly when placing it in the folder.
The main difference in putting skins in Minus from any other pack is that the RSBE01.gct already has all the codes ready for file replacement, and Minus changed the folder path /private/wii/app/RSBP/ to /minusery/ to save time and effort.
If you have any other questions, feel free to ask.