An approximate estimation showed that with about 25 LEDs I could achieve an adequate brightness for working. So 50 LEDs have to be on one PCB, 25 of each colour. An arrangement of 5*5 LEDs (5 in series, 5 rows in parallel each) is recommended, so we get a voltage of ~16V at a current of ~500mA per color.To dissipate the waste heat from the LEDs we need an aluminium core board.
The creation of the layout is a little fiddly, because with an aluminum core PCB a second layer is very expensive. Single layer aluminium core boards cost only about 1.5€ per 100*100mm per piece at 15 pcs. To cross tracks you have to use zero ohm resistors. However, since we only need a few components in a simple sample, this should not be a problem. In addition to the LEDs an NTC is used to measure the board temperature.
Since we want to make the connection between the boards with spacers and an aluminium PCB is made of aluminium from behind, we have to insert an insulation here. Fortunately, spacers and FET-screw-insulators have the same diameter.