Context: The evidence of a multipopulation scenario in Galactic globular clusters raises several questions about the formation and evolution of the two (or more) generations of stars. These populations show differences in their age and chemical composition. These differences are found in old- and intermediate- age stellar clusters in the Local Group. The observations of young stellar systems are expected to present footprints of multiple stellar populations. Aims: This theoretical work intends to be a specific step in exploring the space of the observational indicators of multipopulations, without covering all the combinations of parameters that may contribute to the formation of multiple generations of stars in a cluster or in galaxy. The goal is to shed light on the possible observational features expected by core He-burning stars that belong to two stellar populations with different original He content and ages. Methods: The tool adopted was the stellar population synthesis. We used new stellar and pulsation models to construct a homogeneous and consistent framework. Synthetic color-magnitude diagrams (CMDs) of young- and intermediate-age stellar systems (from 20 Myr up to 1 Gyr) were computed in several photometric bands to derive possible indicators of double populations both in the observed CMDs and in the pulsation properties of the Cepheids. Results: We predict that the morphology of the red/blue clump in VIK bands can be used to photometrically indicate the two stellar populations in a rich assembly of stars if there is a significant difference in their original He content. Moreover, the period distribution of the Cepheids appears to be widely affected by the coeval multiple generations of stars within stellar systems. We show that the Wesenheit relations may be affected by the helium content of the Cepheids. ; Comment: in press on A&A