The bad thing with the light box is that it normally only calibrates at one (unknown) intensity. You must still know about any linearity problems with the sensor. Building a color sensor from cheap hardware not intended for it would probably mean that the sensor datasheet may not show a calibrated curve of output signal as function of luminance.

If the datasheet has info about nonlinearity, the calibration step needs to know where in the curve you are when using the light box, so besides having a cablibrated color, you would also have a need for a calibrated intensity.