Evolution of the colour excess E(B − V) as a function of in the homogeneous and clumpy shell geometries studied in Sects. 3 and 4. The shell geometries are defined by the following parameters: FF = 0.23 and CF = 0.997 (R_min = 49 and R_max = 64 cells), n_IC/n_C = 1.0, 0.01, and 0. Two different colour excess are shown in this figure: firstly, E(B − V)_real, which corresponds to the exact value of the colour excess because estimated from the original definition of the colour excess (from the V and B bands). Secondly E(B − V)_Calzetti, which is estimated from both the attenuation law of “Calzetti” (Calzetti 1997) and the UV escape fraction. An observer would instead measure E(B − V)_Calzetti in practice.