(upper panel) V, R, and I SBPs of I Zw 18. The thick-solid and thin-dashed lines show linear fits to the V SBP in the radius range 7′′  ≤  R^⋆  ≤  15′′ and R^⋆  ≥  7′′, respectively. The effective radius R_eff and the radius R₈₀ enclosing 80% of the total V emission are indicated. The V SBP derived by P02 after subtraction of the Hβ and [Oiii]λλ4959, 5007 emission lines (referred to as V′), shifted by +1 mag for the sake of clarity, is included for comparison. (lower panel) V − R (filled circles), V − I (squares), and R − I (open circles) color profiles, computed from the SBPs in the upper panel. The vertical bar indicates the V − I color range inferred by P02 from ground-based and HST PC data for region ω at the southeastern tip of I Zw 18 (V − I(ω) = 0.17 ...0.32 mag). The mean V − R color of I Zw 18’s host after subtraction of nebular line emission (V′ – R′ = 0.12  ±  0.04 mag; P02) is indicated by the horizontal line. The colors V − I(ω) and V′ – R′ of the stellar host galaxy are by, respectively,  ≳ 0.8 mag redder and  ≈ 0.4 mag bluer than the colors of the exponential nebular envelope (R^⋆  ≥  6′′).