To limit it to a single channel, change $active to the channel name.

Here's a little re-write
Code:
on *:input:#channel:{
  set colorinput $iif($1 == !on,$true,$iif($1 == !off,$false,%colorinput))
  if %colorinput {
    if (/* !iswm $1) {
      if ($network == Undernet) {
        msg $chan 02,03 $+ $1-
        halt
      }
    }
  }
}