The color solid includes all colors perceived by the human visual system associated to physical color stimuli. The optimal or MacAdam colors define its frontier. However, the MacAdam limits, and therefore the shape and volume of the color solid, depend on the illuminant or real light source, even in a uniform color space. In general, the greater the volume of the color solid, the greater the number of distinguishable colors, that is, better colorimetric rendering or quality index. In this work we show two methods to estimate how many distinguishable colors are inside the color solid, particularly using constant lightness planes. The first method fills each constant lightness MacAdam loci in the CIECAM02 chromaticity diagram by squares with unity area. The second method uses the Krauskopf & Gegenfurtner's discrimination model, which permits to fill the constant lightness MacAdam loci with discrimination ellipses increasing in area with increasing distance from the achromatic point. In this way, accumulating the computation of the distinguishable colors for each constant lightness plane, we can estimate the total number of distinguishable colors, so we can establish an absolute ranking of colorimetric quality or color rendering, unlike the CIE color rendering algorithm. Applying both methods for the illuminants A, C, D65, E, F2, F7 and F11 and the real light sources HP1-3, the first position is for the illuminant E, followed by the illuminants C, D65 and F7, and the last positions of this comparative are for the real light sources HP2, HP3 and HP1.