Color–color diagrams u′g′K_s (top panel) and u′g′i′ (bottom panel) including PEGASE.2 population-synthesis models (Fioc & Rocca-Volmerange 1997). The layout is the same as in Fig. 2. In the top-panel diagram, the thin blue lines represent sequences of old single-age stellar populations at redshift zero; the corresponding metallicity range is indicated in the legend. The large colored symbols show the redshift evolution of observed colors for four galaxies formed at z = 3, each characterized by a different star-formation history: burst plus low star-formation rate (squares), constant star-formation rate (stars), exponentially declining star-formation rate (triangles), and burst followed by passive evolution (elliptical; circles).