PACS color–color diagram showing the 24 μm-dark cores (red) and 24 μm-bright cores (black). All sources with high luminosities (L > 10³   L_⊙) are shown in green. The typical error is plotted in the lower-left corner. The right panel shows the distribution of [70] – [100] colors, and the upper panel shows the distribution of [100] – [160] colors for the 24 μm-bright cores (red), 24 μm-dark cores (black), and (in blue) cores for which no counterpart at 70 μm was detected.