Left: Distribution of IllustrisTNG galaxies across the NUVrK colour-colour diagram, coloured according to the classification defined in Sect. 3.1; contours enclosing 68% and 95% of the ageing and retired galaxies are displayed as red and blue lines, respectively, whereas the quenched population is also highlighted by a green shaded area. Black lines indicate the demarcation criteria adopted by Moutard et al. (2018). Vertical and horizontal histograms show the marginal distributions of each class in terms of (NUV − r) and (r − K_s), respectively. Right: Same galaxy sample shown in the sSFR₈ versus sSFR₉ plane, with colours corresponding to the six-class scheme of Moutard et al. (2018); young (Y) / old (O), quiescent (Q) / green valley (GV) / and star-forming (SF). The yellow shaded area highlights the young quiescent (Y-Q) population. Vertical and horizontal histograms indicate the marginal distributions in sSFR₈ and sSFR₉, respectively.