Have your in game name be any mix of colors you want as long as it doesn't make you look like a staff member. (You can't ask for anything that will look like solid Dark Red (&4), Red (&c), Magenta (&5), hot pink (&d), or cyan(&b).
Staff will need to change the color for you. You can change it as many times as you like, but frequent requests may be turned down by busy staff.
/colors will show you the colors available, with codes. Or you can use /cmi colorpicker to see hex colors. It'll be simpler for staff if you tell them the name with the necessary codes in it! (Ie, for a name that's half blue and have red, you could tell us &4Mom&9shroom, and, NO, we are not going to let you nick yourself Momshroom in any color xD)
This includes a nickname even if you don't otherwise have one.