RGB isochrones from the PARSEC set (Bressan et al. 2012; Chen et al. 2019) in the (v − y)₀ − m_1, 0 plane (left panel) and in the (b − y)₀ − m_1, 0 plane (right panel). The five bundles of three isochrones of different ages (2, 7, and 14 Gyr) are labelled with their global metallicity [M/H] = 0.0, −0.5, −1.0, −1.5, −2.2 (from top to bottom). The plot in the left panel is limited to the (v − y)₀ colour range in which the photometric metallicities are actually computed. In the right panel are plotted only the part of the isochrones whose (v − y)₀ colour is in the permitted range, while the range of validity of the H00 calibration in (b − y)₀ is enclosed between the two grey vertical lines.