Dependence of the color correction factors f_c on the relative luminosity for various NS atmosphere models. The f_c are obtained using the first fitting procedure. Top panel: dependences for hydrogen and solar H/He mixture with Z = 0.3   Z_⊙ models with different surface gravities log g = 14.0 (solid curves), 14.3 (dotted curves) and 14.6 (dashed curves). For clarity, the dependences for hydrogen models are shifted up by +0.2. Bottom panel: variation of f_c on chemical compositions: pure hydrogen (upper curve), pure helium (lowest curve), and solar H/He mixture with Z = Z_⊙ (solid curves), Z = 0.3   Z_⊙ (dotted curves), Z = 0.1   Z_⊙ (dashed curves), Z = 0.01   Z_⊙ (dot-dashed curves) for low gravity log g = 14.0 models.