The B − R colour indices as a function of R magnitudes during the 18 nights in the right hand list. The top panel contains 1050 colour index points. To clarify the colour variability, in the bottom panel the and values in each night are reported instead of the dense points, while the bars indicate the variable range of B − R and R. The arrows indicate the time sequence. In both panels, the red dashed lines indicate the average B − R colour index (1.62).