Over on Apartment Therapy DC, the picture in this post New Options for Kid-Friendly Upholstery sparked a comment from Mei-Ling (and a follow-up comment from Home Body). My dad was the first person I knew to use Sharpies this way. Like Mei-Ling, when the carpets in my parent's home began to fade from wear and tear, my father, who was an antiques dealer, brought home an array of Sharpies and had at the rugs with the pens. The porous fibers easily absorbed the inks.
Afterward, instead of looking like they'd been scribbled on (the family fear when he began this project), the rugs looked fresh and bright. The trick is to press the soft tip gently into the rug and let the fibers slowly absorb the ink. Use a dotting motion to push the ink into the rug instead of scribbling motion which will just leave the ink on the top layer.
I suggest that, before attempting to use this fix on your own rug, test how it out in a small portion of your rug, especially if your rug is a synthetic. Not all rug fibers absorb ink the same way.
Image: Abigail Stone