Colour diagram with respect to V band photometry. Red (HOYS) and orange (AAVSO) dots represent V-I, green (HOYS) and light green (AAVSO) dots represent V-R, and blue (HOYS) and light blue (AAVSO) dots represent B-V. The solid coloured lines represent the best linear fit to the data, while the black arrows represent the extinction correction for A_V=1.0 mag and R_V=3.1. The black symbols denote the mean measurements during the days of the PENELLOPE observations.