SB (V − K_s)–color relations considering either B- (top panel) or O-type (bottom panel) stars from our sample. As the reddening is the main contributor to the uncertainties, the error bars are plotted along the reddening vector. For polynomial fits, the relation is extended with redder DEBs (open circles) from D. Graczyk et al. (2021). The linear fits to our sample in the color range [−0.9, −0.6] mag are shown as a red line. The fifth-order polynomial fits (to 62 components in both cases) are shown as blue lines, and the corresponding residuals are shown in the bottom panels. For B-type stars, the linear fit is tangent to the polynomial fit. For O-type stars, the polynomial fit yields significant systematic deviations. Our measured values of S_V and ﹩{(V-{K}_{{\rm{s}}})}_{0}﹩ are available in the electronic version.