Correlation between the HST F336W (∼U-band) and F555W (∼V-band) magnitudes. In order to fit the linear correlation (green line), we exclude stars with F555W fainter than 19.5 (see Sect. 3.1) and brighter than 16.5 (gray regions). Stars that are brighter in F555W but not in the UV are RSGs and therefore not included in the fit. The relation is used to convert F336W magnitudes into F555W magnitudes for all stars in the HST input catalogs. The gray dotted line indicates the one-to-one relation.