Contribution to the diffuse γ-ray emission from a synthetic population of YMSCs in ROI1 (top row) and ROI2 (bottom row) for the three different cases of the diffusion coefficient considered: Kolmogorov (left column), Kraichnan (central column) and Bohm-like (right column). The spectrum is calculated after applying the mask 𝒜 ∪ ℬ. In each plot, the dashed line represents the median flux per energy bin after considering 100 different realisations of the Milky Way population of YMSCs, while the associated shaded region encloses the 25–75 percentile flux. The striped region instead encompasses the 25–75 percentile flux when WR stars are not included in the stellar mock populations. The violet solid lines in the right column and their associated shaded regions show the median and the 25–75 percentile diffuse γ-ray flux after the addition of emission from the CR sea.