Abusively used by newbies and useless mIRC scripts, the mIRC colors are nowadays supported by most IRC clients. Added in version 4.7 (released on 09/12/96), the mIRC colors code are inserted by hitting the ctrl+k key combination. Actually, the symbol used is a UNIX ^C (inversed C). The colors combinations are created this way:

^CF[,B]
Where F is the foreground color and B the optional background color. F and B are numbers from 0 to 15 representing the following colors:
 0 White
 1 Black
 2 Blue (Navy)
 3 Green
 4 Red
 5 Brown (Maroon)
 6 Purple
 7 Orange (Olive)
 8 Yellow
 9 Light green (Lime)
10 Teal (Dark cyan)
11 Light cyan
12 Light blue (Royal)
13 Pink (Fuchsia)
14 Grey
15 Light grey (Silver)
So, ^C4I love apples will print in red, and ^C8,1Eating bananas in the dark is cool will print in yellow with a black background.