Source g − i colour measurement error δC_gi as predicted by inserting the α_i, α_g, Λ_i, and Λ_g (i-band and g-band deviation of the lens light from a Sérsic profile, and i-band and g-band ratio of the lens to source surface brightness measured at the image locations, respectively) of the best-fit models into Eq. (11) (derived correlation between δC_gi, α, and Λ), plotted against the true δC_gi of the best-fit models. The diagonal solid black line indicates were the predicted and true δC_gi are equal. The data point for each simulated system is colour-coded with its Λ_i value. The data points are plotted linearly within the inner olive box.