Two important stellar-population parameters (age and metallicity) of the dominant stellar populations (DSPs) of galaxies are usually estimated by comparing the observed absorption line indices or colors to predictions of some simple stellar population models. However, some studies show that there is actually recent star formation in galaxies, including early type ones. This suggests that we may not be obtaining accurate the two stellar-population parameters for the DSPs of galaxies. This is obvious when we estimate the two parameters by colors, because the youngest populations dominate the light and make the fitted stellar populations younger and richer in metal. We study how young populations (YSPs) in composite stellar populations (CSPs) affect the colors of star systems and to analyze how the stellar ages and metallicities derived from colors possibly deviate from those of the DSPs. It is found that the age and mass fraction of a YSP affect colors of a mixed star system significantly, but the former is stronger. In addition, our results show that the stellar ages and metallicities derived directly from a pair of colors are about 2.14Gyr younger, while 0.0027 more metal rich on average than those of the DSPs of composite stellar systems. Some possible distributions of the differences between stellar-population parameters determined by colors and those of DSPs of CSPs are presented. The possible distributions of the differences between colors of CSPs and those of their DSPs are also shown. Stellar ages and metallicities measured by colors and line-strength indices are compared in the work, with a sample of 18 galaxies. Furthermore, the YSPs may affect the fundamental plane and Kormendy relation of early type galaxies.