Figure 14: color plane for the subsample of LSB galaxies with and colors (black symbols). RC3 galaxies are shown as green symbols. Also shown are 13 Gyrs stellar population tracks (instantaneous burst followed by passive evolution) for a range of metallicities (red) and a 1 Gyr stellar population (blue). Given that the mean metallicity of LSB galaxies is about [Fe/H] = −1.0 , there is no old stellar population that fits their versus colors. Therefore, all indicators are that there must be a significant younger population in LSB galaxies, despite their low stellar density appearance.