Loaded a copy just with:
Click Address Book > Nick Colors > [Enable nick colors] > Add:
1) $me = Orange (Messages and Listbox)
2) [Auto-color based on nick] (Messages and Listbox) (Just for testing, I usually do just Messages on this one)
/server irc.rizon.net (not even SSL, not relevant for this test)
/join a big chan
Press Alt+K > see the nicklist colors all change immediately.
NOTE that they only change ONCE for every mIRC start so you will have to press ALT+K AFTER you join the chan, if you press it before joining the new set or colors will already be defined and mIRC will need to be restarted.