Store the colour code that you want for the nick in the %watchednick.nick. variable, and strip the colour code when assigning the variable to the general %watchednick variable.

I know you're good enough to figure this out, but here's the code, along with a couple of other suggestions
Code:
/watch {
set $+(%,watchednick,.,$1) $+(<ctrl+K>,$$?="colour code",$1,<ctrl+K>)
set %watchednick $addtok(%watchednick,$strip($+(%,watchednick,.,$1)),32)

Code:
on ^*:text:*:#:{
  if $istok(%watchednick,$nick,32) && (kurd !isin $1-) {
  echo -t $iif($target == $me,$nick,$chan) $($+(%,watchednick,.,$nick),2) $1-
  haltdef
  }
}


Any problems, or questions, ask and I'll try to explain, but you're good enough that you should be able to figure out everything.