Color Code GUI Mod 1.8, 1.7.10, 1.7.2 adds in buttons for each of the different color codes available for Bukkit servers. Just hit the button and the color code will be added to your current message! Note that this will not work in singleplayer, as the codes will not be translated into colors.
This mod also included in the mod is a rainbow feature, which will automatically generate rainbow text as you type using color codes!