i'm sorry, but i dont know what you mean by a colormap
If you mean to change the different colors, click on the icon that looks like crayons, or hold down your alt key and click the k key to get there. To read how to use it, in mIRC type
/help colors dialog and another area of the help file that may help is reached by typing
/help control codes which will show you what the colors are for 0 to 15 using ctrl k (assuming you havent changed the default colors)