Comparison of the V − I and R − I colors of I Zw 18 C and of region ω in I Zw 18 with model predictions for a stellar population forming instantaneously (SFH1: thick dotted curve) or continuously, with an exponentially decreasing or constant star formation rate (SFH2 and SFH3: thick solid and dashed lines, respectively). The models have been computed with Pegase 2.0 (Fioc & Rocca-Volmerange 1997) for a constant metallicity (Z = 0.001) and a Salpeter initial mass function between 0.1 and 100 M_⊙, and do not include nebular emission. Thin curves correspond to the same star formation histories but assume an IMF truncated above 5 M_⊙. The vertical bars labeled I Zw 18ω depict the range between the mean and reddest color in region ω of I Zw 18 (V − I = 0.2 ...0.3 and R − I = –0.06 ...0.15). Bars labeled I Zw 18 C (1′′–6′′) indicate the mean color of I Zw 18 C in the radius range 1′′ ≤ R^⋆ ≤ 6′′ ( ≈ 0  ±  0.05 mag). The mean colors and their 1σ uncertainties in the extreme periphery (R^⋆  ≥  6′′) of I Zw 18 C at μ ≳ 27.6 mag/◻″ are depicted by the rectangular areas.