m_F200W vs. m_F115W − m_F200W color-magnitude diagrams of I Zw 18’s main body (left panel) and Component C (right panel). Different stellar evolutionary phases are visible (see discussion in Sect. 3.1). PARSEC-COLIBRI isochrones (assuming [Fe/H] = −1.60, (m − M₀) = 31.3 and E(B − V) = 0.032) are over-plotted and color-coded by their age (see the legend in the upper right corner). The main stellar evolutionary phases are labeled. Error bars on the left-hand side represent photometric uncertainties at various magnitudes, assuming m_F115W − m_F200W = 1. The red-dashed line indicates the 50% completeness limit.