Time variation of the effective temperatures of 0.65 M_⊙ model stars of metallicity Z = 0.001 (top, left panel) and Z = 0.004 (top, right), calculated with different convective models. The corresponding evolutionary tracks of the Z = 0.001 model star on the (F814W−F160W, F160W) and (F110W−F160W, F160W) planes are shown in the bottom, left and bottom, right panels, respectively. The points along the tracks indicate the middle of the different inter-pulse phases. Data points in the catalogue of M31 AGB stars by G22 are shown with grey points and G22 cuts are marked with dashed, magenta lines.