Highest allowed deviation of the lens light from a Sérsic profile (α_i) as a function of the i-band contrast between the lens and source image (Λ_i) to achieve source colour measurements that are more accurate than a set threshold. The solid lines are generated from the derived correlation between δC_gi, Λ_i, and α_i (Eq. (12)). Each solid line shows α_i as a function of Λ_i, for a constant δC_gi. The value of δC_gi increases from 0.01 to 0.1, in 0.01 intervals (from bottom to top). The scattered data points show the distribution of α_i and Λ_i for the best-fit models to the simulated systems. The scattered data points are colour-coded with the δC_gi values of the best-fit models. In agreement with the derived correlation (Eq. (12)), the δC_gi of the best-fit models increases perpendicularly to the solid constant δC_gi lines. This figure indicates that Λ_i is the main parameter that determines the maximum allowed α_i to have δC_gi below a desired threshold.