Hertzsprung-Russell diagrams showing evolutionary tracks with a metallicity similar to I Zw 18. The gray numbers indicate the initial masses of the evolutionary models in solar units. The black ticks indicate the model positions at 0.925, 0.935, ..., 0.995 times the age of the final model, indicating where helium-burning models spend most of their time. The empty region in the CMD of I Zw 18 associated with bright warm sources (log(L/L_⊙) > 5.6 and 7 ≤ T_eff/kK < 20; Fig. 5) is highlighted with an empty green box. The area where we found no bright cool SGs is highlighted with an empty orange box. The areas where we inferred approximately 25 warm and cool SGs are highlighted with filled green and orange boxes, respectively.