The colour indices (B − V) (top panel) and (V − R) (bottom panel) of the total Knot 3 light (corrected for E(B − V) = 0.12, see text) versus its brightness in V band for all available photometric data. The total magnitudes V_Kn.3 of 20.17–20.19 mag correspond to several deep minima on the lightcurve of LBV after the ’giant eruption’. Respectively, the total magnitudes of Knot 3 of V_Kn.3∼19.96–20.03 mag correspond to the maxima of the LBV lightcurve in this period. The solid red lines, overplotted to the both panels, represent the expected changes of Knot 3 colour in the case if the (B − V) and (V − R) colours of the LBV change as a linear function of V(LBV) between quasi S-Dor minimum and maximum. The extremes correspond to an early B (as its (V − I) from the HST data at the epoch of December 2017, with V = 25.0 mag) and to late F near the local maxima, when V(LBV) ∼22.0 mag.