You would have to change the individual colors of your 16 color pallet in mIRC.
You can do this by going to the mIRC Colors control panel, (Crayon Icon or ALT+K) and right-click any of the color boxes to change its value. You can also do this by command, with /color [0-15] [rgb-decimal], and /color -r [0-15] to reset to its default color.
- Raccoon