Colour-magnitude diagram in the field of MACS J0138−2155. Spectroscopically confirmed cluster members and non-members are indicated as blue and red points, respectively. Sources with no spectroscopic confirmation are shown in grey. The blue line and region indicate the fitted red sequence and the 1σ scatter. Red crosses (X) mark cluster members that do not follow the red sequence and yellow plus signs indicate the photometrically selected members that are included in our lens model. Objects inside the red sequence with a red minus sign are not included as photometric cluster members due to the lack of, or unreliable, F160W photometry.