Left panels – m₁ vs. y − K plane [panel a)] for isochrones at fixed cluster age (t = 12 Gyr) and different global metallicities ( [M/H] , see labeled values). The evolutionary phases range from  ≈ 4 mag fainter (M_d, empty squares) than the MSTO to  ≈1 mag fainter (M_u, asterisks) than the MSTO. Evolutionary tracks were computed by assuming α-enhanced chemical mixtures (PI06) and transformed into the observational plane by adopting atmosphere models computed assuming the same α-enhancements. The panels b) and c) show similar relations, but in the m₁ vs. y − H and in the m₁ vs. y − J plane. Right panels – same as the left, but for the m₁ vs. u − y [panel d)], m₁ vs. v − y [panel e)], m₁ vs. b − y [panel f)] planes.