Observed and model colors in the u − g vs. g − r two-color diagram. Top panel: the continuous lines are model values for a temperature sequence from 5400 K (lower right) to 9000 K (upper left) with steps of 200 K. The dotted lines connect the same T_eff for 2 different abundances; the upper line in the right half of the figure corresponds to abundances of SDSS0916+2540, the other line to SDSS1103+4144. The (red) circles are the observed data. Bottom panel: the same models, but with the neutral broadening constant for the Mg ii resonance lines increased by a factor of 10 to simulate a stronger blanketing effect in the near ultraviolet.