Comparison of correlations between absorption line indices and colours derived from the observational data and BC03 model, respectively. Points show the correlations derived from BC03 model. Six kinds of points denote stellar populations wit metallicities Z = 0.0001,0.0004,0.004,0.008, 0.02, and 0.05. Solid lines show the best fits to correlations between observational absorption line indices and colours of early-type galaxies. Dashed lines show the 1σ scatters of correlations derived from observational data. Three panels are for three metallicity sensitive indices, i.e., Mg₁, Mg₂, and Mg_b, respectively.