The colour-matching functions of standard observers proposed by the CIE represent the normal colour vision for the worldwide population. But there are deviations in the colour-matching functions for real observers with a normal colour vision, so the observer metamerism index was defined by CIE to evaluate the mismatch between them in colour appearance. In this work, we present an alternative form to evaluate the differences between the CIE standard observer and real observers (Stiles-Burch database) according to the number of distinguishable colours in the colour solid associated to each observer. Unlike the metamerism index defined by CIE, we evaluate globally the colour appearance for a real observer taking into account its colour gamut volume. After analyzing the results, we have seen that the gamut volume of the real-observers is lower than to that associated to the CIE standard observer, even to that associated to the CIE modified observer. Therefore, this work and its methodology could be used to know the ability of the CMF's of different observers to get the maximum colour gamut under the same illuminant. Furthermore, this methodology could be applicable to study the gamut volume variability of the real observers regarding to age, race, etc, or even with abnormalities in colour vision (dichromacy, pathologies, etc).