Correlation between V-filter magnitude and (V-Ic) colour index based on KWS data and two linear fits. One of the fits corresponds to a normal variability, and the other corresponds to the dimming period data. The standard deviation of the fit residuals is 0.03 mag for the pre-dimming period and 0.04 mag for the dimming period. For (V-Ic) colour, KWS has an instrumental uncertainty of about 0.025 mag.