Average observed colors of BL Lac host galaxies compared with those expected from old stellar populations as a function of redshift. Observed V−I colors (squares); converted from R−I (see text; triangle). The solid and dashed lines indicate the expected color for normal elliptical galaxies assuming passive evolution and ﹩q_{0}=0﹩ as reported by Fukugita et al. (1995) and Bressan et al. (1994), respectively.