In the first film (Episode 4: A New Hope), the wizards at ILM used a number of techniques, including some that were practical (performed on the set, rather than added in post). They'd cover a blade in reflective material, and spin it with a motor hidden in the hilt. When lit, it appeared to glow. The colour wasn't particularly vibrant, however, especially from certain angles, and the props were rather fragile; they didn't hold up well during duels. If you watch the film carefully in its original form, you can actually tell when these were used. For later films in the series, and the Special Editions, the effects teams used rotoscoping exclusively. Nowadays, the painting is done by computer.