Similar to the bottom left panel of Fig. 2, but showing only the infrared data of I Zw 18 and zoomed in on the bluer side of the CMD. Here, we show iso-luminosity lines extending to bluer colors. The temperature ranges associated with the color intervals bordered by the vertical black lines are listed in the plot. The green hatching highlights the empty region associated with T_eff < 20 kK and log(L/L_⊙) > 5.6. The red ticks indicate the absolute F200W magnitudes at which Bortolini et al. (2024) find a completeness (f_compl) of 20%, 50%, 80%, and 95% in the densest region of I Zw 18, R1. With black crosses we show typical observational errors from Bortolini et al. (2024) at absolute F115W magnitudes of −8, −7, and −6, also estimated for R1.