  We extend the {\sl\,PARSEC} library of stellar evolutionary tracks by
computing new models of massive stars, from 14\Msun to 350\Msun. The input
physics is the same used in the {\sl\,PARSEC}~V1.1 version, but for the
mass-loss rate which is included by considering the most recent updates in
literature. We focus on low metallicity, $Z$=0.001 and $Z$=0.004, for which the
metal poor dwarf irregular star forming galaxies, Sextans A, WLM and NCG6822,
provide simple but powerful workbenches. The models reproduce fairly well the
observed CMDs but the stellar colour distributions indicate that the predicted
blue loop is not hot enough in models with canonical extent of overshooting. In
the framework of a mild extended mixing during central hydrogen burning, the
only way to reconcile the discrepancy is to enhance the overshooting at the
base of the convective envelope (EO) during the first dredge-UP. The mixing
scales required to reproduce the observed loops, EO=2\HP or EO=4\HP, are
definitely larger than those derived from, e.g., the observed location of the
RGB bump in low mass stars. This effect, if confirmed, would imply a strong
dependence of the mixing scale below the formal Schwarzschild border, on the
stellar mass or luminosity. Reproducing the features of the observed CMDs with
standard values of envelope overshooting would require a metallicity
significantly lower than the values measured in these galaxies. Other
quantities, such as the star formation rate and the initial mass function, are
only slightly sensitive to this effect. Future investigations will consider
other metallicities and different mixing schemes.
