Metallicity colour gradients of our galaxies plotted against their age colour gradients (see text for definitions). The error bars denote the standard errors (see Sect. 3). The correlation of the errors is displayed by a two-dimensional 1σ Gaussian contour. The colour-coding on the y-axis defines the ranges of the four subsamples that all include the same number of galaxies (30). The fraction f of galaxies in the age colour gradient distribution is presented in the right histogram.