Evolution of the spectral slope of the optical/NIR afterglow, β. Open points are either derived from UVOT (plus TAROT, REM in the first epoch) or from joint UVOT+GROND fits. Closed points are GROND-only. See Table 5 for the data. Red points are a composite R_C -band light curve of GRB 111209A (identical to the one given in Figs. 10 and 11), added for purposes of orientation. The regions are: I. Early prompt emission. II. First afterglow-like decay. III. Plateau. IV. Rebrightening. V. Late afterglow. The plateau/rebrightening shows a marked rise of the spectral slope, from ≈0.95 to ≈1.25, where it then remains essentially constant. The two different behaviours themselves can be fitted linearly (in temporal log-space), the rising phase has a slope of 2.25 ± 0.37 (dashed blue line), while the constant phase has a slope of −0.14 ± 0.06 (dotted green line), indicating some low-significance chromatic evolution may still be present. Data beyond those shown here show a significant departure from a power-law form due to the increasing dominance of the supernova component.