After obtaining a suitable lamp, I measured it in its four color modes: Red LED only, Blue LED only, Green LED only, and all together (white) respectively.
As expected, the sum of the individual luminosities of the colors should give the luminous intensity of white. This is not quite the case in the curve shown here.
This correlation was not confirmed for the photometer with data logger, because this device (which is intended for measuring UV radiation from screens) is almost blind in the red and detects only blue and green (or their share in the respective mixed color).