You can also try using Boltbaits and Illnabs gradient plugin. Select the letters you want, then select the same color for the primary and secondary colors (It cannot be black, white, or gray. Also, if you click on the more button on the colors window on the bottom right, Under RGB, you want one of those values to be at or around 255.) Then, use the gradient plugin and check the bubble that says HSV long. That will create a rainbow effect.