Lens light residuals can have non-negligible colour gradients. This figure shows the distribution of the g-band and the i-band deviations of the lens light from a Sérsic profile (α_g and α_i) for the systems in our simulated sample. The dashed line indicates where residuals have no colour gradients (i.e., where α_g = α_i). The data is plotted linearly within the inner olive box. This figure shows that the deviations of the lens light from a Sérsic profile in different bands are not necessarily correlated.