You may know this better as Spray-On Hair, or perhaps Hair In A Can. This is another fine product from the Ron Popeil family of inventions, which includes the Showtime Rotisserie Grill and the Pocket Fisherman. How does it work? Just spray it on your balding scalp, and then comb whatever hair you have over the spot, giving the illusion of thick, lustrous hair. Ron himself has demonstrated GLH (which stands for Great Looking Hair) on numerous infomercials for his other products. Afterwards, it looks more or less like he spray painted his head, but GLH is still referred to as "fantastic" and "revolutionary".
The pigment is water-soluble, and thus will wash out easily. According to the FAQ at ronco.com, you shouldn't wear a light-colored hat while GLH is on your head, as the pigment will probably come off. It is also safe to use on your dog, if your dog is missing hair (I swear I'm not making this up). Comes in 9 dazzling colors, ranging from white to blonde to many shades of brown to black.