Absolute visual magnitudes (narrowband color as defined by Smith 1968) versus spectral subtype number for the putatively single stars in our sample. Colors code the absence (green/light) or presence (red/dark) of hydrogen. While small symbols represent individual stars, the thick symbols mark the average M_v of each subtype. The thick lines indicate linear fits to the early (WN2–5) and late (WN6–11) subtypes, respectively.