Correlation between the values of the log ₁₀B_M parameter for the blue and red filters. The bold line is the diagonal, while the dashed line is the standard least squares bisector line. The dot-dashed, dotted, and dot-dot-dot-dashed line correspond to the robust optimal M-regression line, the robust mean biponderate regression line, and the robust winsorised regression line, respectively. The SA galaxies are represented as filled circles, the SAB galaxies as open circles, and the SB galaxies as open stars. The upper panel gives results for the OSU sample and the lower panel for the Frei sample.