HR diagrams color-coded as a function of the highest fraction of turbulent pressure as derived from a best fit of the tracks in Fig. 1 (central panel, LMC metallicity), Fig. 2 (right panel, SMC metallicity), and from Fig. 1 of Grassitelli et al. (2015a; left panel, MW metallicity). The values of the expected macroturbulent velocities () given in the top bar were obtained by adopting the linear relation between the observationally derived macroturbulent velocities of a sample of Galactic OB stars and the corresponding highest fraction of turbulent pressure from Fig. 5 of Grassitelli et al. (2015a).