The following information is taken from Merlin's 'mIRC.ini Unleashed' (a very handy resource)
The lastest version, available from MISHScript , is for mIRC v6.17.
But these settings seem to be the same in mIRC v6.21
They are in the mirc.ini, albeit cryptically, under the [options] heading.
The values are from 0 to 15, corresponding to your mirc color palette.Events color: n4 - position 31
Message color: n4 - position 28
Highlight color: n6 - position 31