Long-term variations of HR 5171A in the four colour indices (in logarithmic scale) of the medium broadband VBLUW system 1980–1991, showing a slightly undulating reddening trend for the V − B (equivalent to the (B − V) of the UBV system), B − L and B − U colour indices. Blue is up. The U − W curve behaves differently, showing two oscillations due to Balmer continuum brightness changes with a timescale of about 2000 d. The last maximum (∼JD 24 47600) has an amplitude of about . This maximum and the preceding minimum might be related to a local maximum or plateau and a minimum, respectively, in the K light curve (thus, also likely present in the other NIR bands). Bars labeled with “JHKL-etc” in the (B − U) panel will be discussed in the next paper.