Colour evolution of SN 2024bch compared to those of selected transients that show high-ionisation features in their early spectra. Objects were selected from those with a similar evolution in their main features (including the duration of the narrow features and SN type) with data available in the literature. In the upper panel, the evolution of the U − g colours for SNe 2017ahn, 2020pni, and 2022jox was included, since u-band photometry for these objects was not available. U-band magnitudes for these objects were converted in the AB photometric system adopting the Vega – AB magnitude conversions reported in Blanton & Roweis (2007). In the same panel, the inset includes a zoom-in of the u/U − g early evolution, which shows a rapid decrease in the colours within approximately the first 3 days after explosion for all the selected objects (see the main text). Phases refer to the estimated explosion epochs reported in the literature. Light curves of SN 2023ixf were collected from Li et al. (2024) and Zimmerman et al. (2024).