SNe Ia absolute magnitudes, M_B, as a function of SNe Ia colour C. Black circles are ‘normal’ SNe Ia and red squares are reddened SNe Ia. The purple line represents a linear fit to all SNe Ia. Open symbols indicate objects for which their metallicities were adopted from a proper gradient estimation, whereas open symbols show objects for which the metallicity was assumed to be the same as the nearest H ii region.