Example of using two-color magnitude diagrams to identify the red sequence of a galaxy cluster at z = 0.28 (cluster 100320). Top and bottom panels show g′-r′ and r′-i′ versus z′-band magnitude, respectively. Gray points are all galaxies within the radius of 0.5 Mpc from the X-ray source center. Black dots are red galaxy group members selected according to the method described in Sect. 4.1. The upper and the lower ranges of colors are shown with the horizontal dashed lines.