My suggestion... not for the regsubex' sake! but
- with .msg others will see the colors - you don't
- without a halt, you'll send the text twice (although you see only the one mIRC processes - the custom message was .silenced)
- with single-digit color numbers you can't type digits properly
- the $window().type thingie prevents e.g. messages to custom windows

Code:
on *:input:*:{
  if (/* !iswm $1) && (!$ctrlenter) && ($istok(channel.query.chat,$window($active).type,46)) { 
    msg $active $$regsubex($strip($$1-),/(\S)/g,$+($chr(3),$replacex($calc(\n % 3),0,12,1,04,2,00),\t))
    halt
  }
}