As for original question. /help highlight
Props tp DJ_Sol for this answer. To change the nick colours in the nicklist (it's well concealed!): Tools -> Address Book -> click on Nick Colors...
See what I mean? It isn't until you realise these things are there, that you realise mIRC isn't so well designed... It isn't even specified in the "help" section for "nick colors" how to get to this dialog! Which reminds me, an explanation of this dialog is available in the help file: /help nick colors