alias rx { 
  :redo | var %x $r(2,15) , %y $r(2,15)
  if (%x = %y) goto redo
  if ($1) return $+(,%x,$chr(44),%y,)
  else return $+(,%x,)
}
alias rc return $regsubex($1-,/(.)/g,$+($iif($prop,$rx(1),$rx),\1))

Useage:
//echo -a $rc(This is a test 12345) <- Random color no background color.
//echo -a $rc(This is a test 12345).whatever <- Random color + Random bkg color.