Reference for color sensor
You need a light source with a spectrum of known power vs. color (i.e. frequency or wavelength) distribution.

(As much as I understand your problem you don't need absolute intensities (e.g. Watt/Steradian) but just relative intensities with respect to color).

There come two relatively simple sources to mind:


black body radiation (e.g. of a incandescent light bulb) whose spectral distribution is directly known by a physical law, Planck's law (you need to know the temperature of the filament)
sunlight (after passing through the atmoshphere till e.g. sea level) whose spectral distribution is empirically known (well measured) and you have a high chance of finding published spectral distribution data.