We explain a method for calculating the optimal sampling interval of color spectra. The 1269 measured Munsell matt reflectance spectra set is used as the test set. The effect of light sources on the required sampling interval with different types of spectra is studied. It is shown that a 20 nm interval is enough for the smooth Munsell set alone, but 10 nm is not enough for the same set matched with a fluorescent light source. However, 5 nm is shown to be enough in most situations.