This figure illustrates the procedure to determine the rMS fiducial line. A region of the CMD focused on the triple MS of NGC 2808 is shown in panel a). The continuous line is the first-guess fiducial for the rMS, drawn by hand. Panel b). The same CMD, after subtracting from the colour of each star the corresponding colour on the fiducial at the same magnitude. Right panels c) and d) show the colour distribution of stars in the 0.25 F814W magnitude intervals quoted in the inset. The continuous black lines are least squares fits of a sum of the three Gaussians that are represented as blue, green, and red lines. The Δ(m_F475W − m_F814W) values corresponding to the peak of the red Gaussian and used to derive the red MS fiducial line are superimposed on the CMD of panel e) (red filled circles). For completeness, in the latter panel we also show the fiducials for the bMS (blue triangles) and the middle MS (green squares).