the best way is to take an existing helmet, and edit it or simply paint over it. open the tex file for the helmet in something like GP4Master, extract the file so you get a .tga file, then open that file in somehting like PaintShop Pro or Photoshop, paint over it, save it, then import is as a tex using GP4Master again, and recompress the wad, and it should appear in-game.
RIP Jules, never to be forgotten. #KeepFightingMichael