Seven sequences of dual metallicity models covering a wider range in metallicities. The metallicity contrast between Z_B and Z_A is fixed at θ = 1.0 dex. Each sequence corresponds to a different metallicity Z_A value, which is indicated in the line color legend in Z_⊙ units. An open circle along each sequence indicates the position where the relative weight applied to each model is equal (w_i = 0.5). In all the above sequences, the ionization parameter is U = 0.01, and the SED corresponds to the zero-age instantaneous starburst model with M_sup = 100      M_⊙.