Light curves and spectral behaviour of OJ 287 at the time of the GR flares. Top panel: light curves of 37, 97.5, and 343.4 GHz bands from the period MJD 57300–57550. To construct the spectra, the data were re-binned with a weekly cadence and averaged. Highlighted data points show the selected averaged bins of each light curve. The dotted red line shows the time when K was within the 86 GHz core. The two dashed blue lines show the times of the 2015 GR flares. Bottom panel: time evolution of the spectrum. The order of time is colour-coded based on the coloured patches in the top panel.