At the moment, there is no simple way to simulate the default mIRC nickname coloring when modifying the on text or on action event, even though there are various identifiers which relate to nickname coloring.
We came to the conclusion in
this thread.
The main problem is that $nick($chan,$nick).color returns the "default text" color when a nickname does not match any entry in the colors list. This messes up the "action" color in on action events aswell as highlight colors etc.. If it would just return $null, it would not be a problem.
I hope there will soon be a solution for this, because it really can't be that I can't even have the same nickname coloring in my theme engine than there is in normal mIRC.