I'm going to start off saying this might be due to windows but here goes:
if you use in mirc.ini:
[colors]
n0=black,1,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,1,0,1,0,15,0,1,1,0
There are some issues from highlight text/list box items in the mirc window (treeview, nicklist) and the edit box. but this issue does not effect the chat windows at all.
if you use the colors setting above, and highlight something, the highlight color will default to windows default highlight color but there will be an issue making it hard to see exactly what is being highlighted, if you notice though, the chat windows are not effected which leads me to believe that this is a bug in mIRC, not windows (but read my opening statement!)
I've tried my best to explain this, hopefully it will be fixed.
screen shot in action:
http://e-motz.com/uploads/mirc_highlightBug.png