Variation of QSO color indices with redshift

In this paper we discuss the effect of features in the spectrum of QSOs on their color indices and the consequent variation of the latter with the redshift. As the redshift increases, the various absorption features shortwards of the Ly emission line enter into the visible region and alter the way in which the color indices vary with the redshift, while the effects of the intrinsic power-law spectrum and emission lines on the color indices become secondary. The largest effect is due to the Lyman limit absorption system. These results are checked using the color indices calculated from IUE spectra.