Optical-NIR, color−color diagram, B − I vs. I − K_S of the integrated colors of the PRGs and their neighbor galaxies shown with labeled red squares. Different gray dashed-lines show the age-labeled isochrones with increasing age from bottom to top for all metallicities in the Bruzual & Charlot SSP models as indicated in the figure legend. The typical photometric uncertainty of the galaxies’ colors, given by SDSS and 2MASS, is about 0.2 mag, and is shown in the bottom right.