It depends on what client you use.
In most irc clients, you can use colors by typing ctrl +k , for bold ctrl + b, for underline ctrl +u
In the icq flash chat client on www.icq.com you have to do color on the site: hit and hold ALT then press 03 on the number pad on the right, then let go, and type: for white - 1 for black - 2 for dark blue - 3 for dark green - 4 for red - 5 for brown - 6 for purple - 7 for orange - 8 for yellow - 9 for bright green - 10 for teal - 11 for bright blue - 12 for blue - 13 for pink - 14 for dark grey - 15 for light grey. Also in stead of 003 you can type 002 = bold and 031 = underline.