  Some studies show that there are recent star formations in galaxies including
early type galaxies. This suggests that we possibly did not obtain the accurate
ages and metallicities of the dominating stellar populations of galaxies from
colors. We study how young populations (YSPs) in composite stellar populations
affect the colors of star systems and analyze how the stellar ages and
metallicities derived from colors possibly deviate from those of the dominating
populations of composite star systems. Some possible distributions of the
differences between stellar ages and metallicities determined by colors and
those of the dominating stellar populations of composite stellar systems are
presented in this work. Furthermore, some possible distributions of the
differences between colors of composite stellar populations and those of their
dominating stellar populations are also shown in this paper. In addition, we
compare the stellar ages and metallicities determined respectively by a color
pair and two line strength indices. The results obtained by a color pair and
those obtained by a few independent colors are also compared in the paper. We
also discuss about how the YSPs may affect the Fundamental Plane and Kormendy
relation of early type galaxies. The main results can be downloaded from the
CDS.
