You need BrawlBox to open up a character's texture files, a good picture editor such as paint.net or the GIMP, and Luigi's original files.
Brawl Box:
http://forums.kc-mm.com/Gallery/BrawlView.php?Number=35494
Where to find Luigi's original files:
https://drive.google.com/folderview?id=0B4vhNdf69qaSZ0N0MnBjOXFYMU0#
Go to Fighter>Luigi>FitLuigi03.PCS(which is the blue costume) and download it. Open up the pcs file through BrawlBox and export the texture data for his hat & edit to your heart's content.
When done editing his textures, put them back inside the pcs file.
Move the pcs file to his folder on the SD card and Luigi should have a new blue texture edited by you.