The farbwerk need the 5V power to run.
The farbwerk is designed to control LEDs with a voltage between 0 and 24 V. When installed to a PC system, the power supply will be 12 V so the LEDs or strips must be designed to use this voltage. Outside the PC and with an appropriate power supply unit, it is also possible to use 24 V LEDs.
Does this mean that external power suply should provide 24v and 5v? (I have never seen such power supply)