Color–color diagrams, (u′ − g′)0 vs. (g′ − i′)0 (top panel) and (u′ − i′)0 vs. (i′ − Ks)0 (bottom panel), showing as gray dots all of the NGFS central sources in the R < Rvir/4 with PSF photometry. The red solid circles represent the nuclei of this work, and the blue diamonds show the confirmed CSSs in the surroundings of the cD galaxy NGC 1399. Compact stellar systems such as GCs, UCDs, and nuclei lie in a very narrow region in color–color space, which makes the u′ g′i′ and u′i′Ks diagrams very powerful tools to select CSS candidates.