Color vs. magnitude diagrams showing the SSP curves of the 13 metallicities used in the model grid, with discrete model distributions of age log ₁₀(t/ yr) = 7, 7.5, 8, 8.5, 9, 9.5, and 10 (young models are top left and old models are bottom right in each panel), for three metallicities: Z = 0.019 (red), 0.005 (orange) and 0.00019 (blue). Each distribution contains 1000 discrete cluster models with mass fixed to log (M/M_⊙) = 4, and the SSPs are scaled to the mass of the clusters. The reddest SSP line indicates highest metallicity Z = 0.03 and the bluest SSP line indicates lowest metallicity Z = 0.00013. The 1-σ (σ = 0.15 mag for FUV and NUV, and 0.05 mag for U and B passbands) error bars show the standard deviations of the photometric accuracy. The black arrows indicate the extinction vector, computed for A_V = 1 mag. Panel a) shows the situation in optical bands alone, while in panels b) and c), ultraviolet information is present, and the zoom plots show details at young age.