A contour plot showing the distribution of galaxies with a mass 10^10.25   M_⊙ ≤ M_∗ ≤ 10^12.0   M_⊙ within the u^∗ − r^∗ vs. D_n4000 plane. The colorbar on the right indicates the number of galaxies per bin of width 0.025 in both u^∗ − r^∗ and D_n4000. If the number of galaxies per bin is less than 5, these galaxies are shown as black dots instead. The vertical red line indicates the optimum separation in the Petrosian color, u^∗ − r^∗ = 2.22, proposed by Strateva et al. (2001). Although this does indeed separate galaxies of the blue cloud and red sequence, the separation between the two populations is more pronounced in D_n4000. Therefore the horizontal separation lines D_n4000 = 1.45 and D_n4000 = 1.7 are adopted in this research. Galaxies between these two values are classified as green.