Warming up or cooling down one side of a face, turning a bland white conference room wall into a pleasing deep blue or powerful red is just a sheet of polymer away. The biggest downside to gels for small portable flashes is that they are usually manufactured in large sheets, sometimes 12" square or larger. This can be overkill when looking to cover a flash head that is just a couple of inches wide.
Roscolux or Cinegel sample packs, which include a huge variety of colors in a 1-1/2" x 3-1/4" size, are a good solution. Just tape the sample sheets directly onto the flash head, and then put a modifier, like Stofen, over the head to help hold the gels in place. The resulting color is nicely saturated These sample packs can usually available for the price of shipping at stores like B&H Photo or some cinematographic houses if you're lucky enough to have one locally.
(image via flickr user g2boojum)