This is a fairly quick script I made, which assigns each person in the channel a random colour for their nick.
Obviously due to mIRC's limited colour range, there will always be conflicts.. but at least it gives you some indication, and it is easier to find nicks if you're scrolling up.

Just to explain a couple of things.. firstly, I've made it exclude colours 1 and 2, as these are particularly dark, so would not work on my black mIRC background. You should change this as appropriate for whatever colour background you have.
Secondly, I have made it so it does not activate the script when you are highlighted, because mIRC highlights WILL NOT WORK simultaneously with the script.

Finally, remember that this script uses variables, so if you like to keep a clean variable list then this isn't for you. I'm sure there's a better way to do it, but I did this more for personal use. If anyone wishes to have some input in making this better, feel free to do so in the comments.

Hope it works well for you..

EDIT 26/10/2011: Updated so the channel name still lights up on new messages in the tree etc. (Thanks to GrimReaper for the fix!)

on ^*:TEXT:*:#: {
  if (!%nickcolour. [ $+ [ $nick ] ]) {
    while ( !%nickcolour. [ $+ [ $nick ] ] || %nickcolour. [ $+ [ $nick ] ] == 1 || %nickcolour. [ $+ [ $nick ] ] == 2 ) {
      set %nickcolour. [ $+ [ $nick ] ] $rand(0,15)
    }
  } 
  if ($me !isin $1-) {
    haltdef
    echo -t $chan $+(<, $+ %nickcolour. [ $+ [ $nick ] ] $+ ,$iif($regex($nick(#,$nick).pnick,/([~&@%\+])/),$regml(1)),$nick,,>) $1-
    window -g1 $chan
    halt
  }
}
on ^*:ACTION:*:#: {
  if (!%nickcolour. [ $+ [ $nick ] ]) {
    while ( !%nickcolour. [ $+ [ $nick ] ] || %nickcolour. [ $+ [ $nick ] ] == 1 || %nickcolour. [ $+ [ $nick ] ] == 2 ) {
      set %nickcolour. [ $+ [ $nick ] ] $rand(0,15)
    }
  } 
  if ($me !isin $1-) {
    haltdef
    echo -t $chan $+($chr(42),$chr(0160), $+ %nickcolour. [ $+ [ $nick ] ] $+ ,$iif($regex($nick(#,$nick).pnick,/([~&@%\+])/),$regml(1)),$nick,, ) $1-
    window -g1 $chan
    halt
  }
}