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For large systems, an analog approach may prove better. Imagine a scenario in which a single "control module" is sending three PWM signals (R,G,B) to various (possibly large) LED modules in a daisy chain fashion.

In this case, white balance is done separately in each module by varying the current supplied to each triad when receiving signals of maximum value (maximum duty cycle). Possibly three pots per module are employed to make the adjustment, one per color (RGB).

NOTE:
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Actually, "maximun duty cycle" means DC. So white balance can be done in LED modules individually whithout having to be connected to the "controller board". Maybe, by actuating switch labeled "White" inputs get fed with DC so the adjustment can be done in this conditions.