Upper panel: colour-mass plane for galaxies in the redshift range 0.1 < z < 0.5, above the mass limit log (M/M_⊙) = 10.2. Green dashed lines represent 15%, 50% and 85% of the color distribution, in bins of 0.2 in mass (logarithmic scale, as represented by the horizontal line in the bottom right corner). Black contours are the isodensity contours of the plotted points. The red straight line is the colour thresholds used to define “very red” galaxies. It is a straight cut following roughly the red sequence and 0.3 magnitudes bluer than the red sequence itself. Middle panel: in not-independent bins of 0.2 in log (M/M_⊙) units, thick circles are the fraction of “very red” galaxies in the highest density quartile and thin squares in the lowest density quartile. The horizontal dashed line at fraction = 0.5 is for reference. Bottom panel: the significance of the slope of the linear fit of the red fraction as a function of density, in each mass bin. In these panels, density is computed with fainter volume limited tracers and without mass weights.