Color distribution of galaxies within $r_{200,\mathrm{wl}}$ (red histogram) and in a control area (normalized to the solid angle of the cluster, blue histogram) for O19. Only galaxies brighter than M_V^e = −20 mag are considered, assuming all galaxies are at the O19 redshift. The vertical lines indicate the expected color range of the red sequence. The single peak in the color distributions confirms the absence of a substantial contamination by other structures, as shown through spectroscopic data and the color-magnitude distribution in Paper II.