Vacuum the rug to remove dirt and dust.
Stick painter's masking tape to your rug to outline the areas you want to highlight with new color. You can also apply stencils to the surface for more complex designs, though a typical area rug "highlight" is just a small strip of extra color around the edges.
Paint inside the border of your painter's tape or stencils using fabric paint and a 1/2-inch artist's brush. The tape is meant to be a guide, but it won't protect against sloppy excessive fabric paint. Rugs are naturally nappy, and the tape doesn't stick well enough to the surface for good protection. To avoid painting outside the taped boundaries, a 1/2-inch artist brush gives you precise paint control. Thinner brush sizes are also available for highly detailed jobs. Artist's brushes are sold at art supply stores, and fabric paint is found at craft stores.
Remove the tape after the paint has dried for a full 24 hours. The paint will be dry to the touch beforehand, but most paint products do not fully cure for an entire day.