Code:
alias hexrgb { 
  tokenize 44 $iif($0 > 1,$+($calc($1),$chr(44),$calc($2),$chr(44),$calc($3)),$rgb($1))
  return $base($rgb($3,$2,$1),10,16,6)
}

That will work for both $hexrgb(16777215) and $hexrgb(255,255,255) inputs.