  We present evolutionary synthesis models applied to near--infrared spectral
features observed in the spectra of young Magellanic Cloud clusters and
starburst galaxies. The temporal evolution of the first and second overtones of
CO at 2.29 micron (2-0 bandhead) and 1.62 micron (6-3 bandhead) and of the
(U-B), (B-V) and (J-K) colors are investigated. We find that the current
evolutionary tracks of massive stars with sub-solar chemical composition in the
red supergiant phase are not reliable for any synthesis of the temporal
evolution of infrared stellar features. The high sensitivity of the selected
infrared features to the atmospheric parameters of cool stars allows us to
place constraints on the temperature and the fraction of time spent in the red
part of the Hertzsprung-Russell diagram by massive stars during their
core-helium burning phase. We derive a set of empirically calibrated
spectrophotometric models by adjusting the red supergiant parameters such that
the properties of the observed templates are reproduced.
