i hope that i can update my arduino tomorrow, so that i can test if this will work....

The FHEM code you want:
The Devices:
define LEDRahmen MYSENSORS_DEVICE 102 ... autmatic including des Reading/Devices
define LEDRahmenSwitch dummy -- for setting LED and Color manually
define LEDRahmenSwitchNotify notify LEDRahmenSwitch.* { sendLEDColor($EVTPART0, $EVTPART1) }

and the 99_myUtils.pm Mehtods:
(NEW: i have changed the Method from send String like 0-255-0 to 000-255-000 - see Method "changeLengthTo" )

sub sendLEDColor($$){
  my ($stat,$val) = @_;
  my $tmp;
  $tmp .= changeLengthTo(hexstr_to_signed32int(substr($val,0,2)),3);
  $tmp .= "-";
  $tmp .= changeLengthTo(hexstr_to_signed32int(substr($val,2,2)),3);
  $tmp .= "-";
  $tmp .= changeLengthTo(hexstr_to_signed32int(substr($val,4,6)),3);

  #my $m = hexstr_to_signed32int($tmp);
  #Log 1, "MeinE Testmethode";
  #Log 1, $;
  fhem("set LEDRahmen status".$stat." ".$tmp."");


}

sub changeLengthTo($$){
  my($str,$toLength) = @_;
  my $tmpstr;
  for(my $i = (length($str)+1); $i <= $toLength; $i++){
    $tmpstr.= "0";
  }
  return $tmpstr.$str;
}

sub hexstr_to_signed32int($) {
    my ($hexstr) = @_;
    die "Invalid hex string: $hexstr"
        if $hexstr !~ /^[0-9A-Fa-f]{1,8}$/;
 
    my $num = hex($hexstr);
    return $num >> 31 ? $num - 2 ** 32 : $num;
}