Source colour measurement error correlates with the lens light residuals. This figure shows the error of the measured source g − i colours (δC_gi), plotted against the residual ratio parameter (ω, defined in Equations A.1 and A.3). The dashed line shows the best-fit line passing through the origin with a −1.867 ± 0.028 slope. The thin grey lines show the ω = 0 and the δC_gi = 0 lines.