Color-space metrics are considered and three main aspects are analyzed: (1) the distinction between small and large color differences; (2) the relation between space metrics and color-difference formulae: (3) the color-scale comparison between CIE 1931 and CIE 1964 observers. The analysis, based on daylight adaptation, is, made for the 2 degrees visual field on a chromaticity diagram with uniform scales according with the MacAdam ellipses' and, for the 10 degrees visual field, on the OSA-UCS.(2.3) The conclusions are: (i) the main difference between 2 degrees and 10 degrees observers is a proper dilation of the chromaticity scales; (ii) the chroma scale of the Munsell system is uniform for the 10 degrees visual field; (iii) the scales of the Munsell system are in good agreement with those of OSA-UCS; (it,) the usual color-difference formulae (CMC, CIE94) represented on the OSA-UCS space show a distortion that seems due to their definition in the CIELAB space; the BFD,formula shows a lower distortion.