Colour of C/2025 D1, measured using a 3 × 10⁴ km radius photometric aperture, compared with various small Solar System body populations (Dandy et al. 2003; Szabó et al. 2007; Solontoi et al. 2012; Jewitt 2015; Markwardt et al. 2023, and citations therein) and the Sun (Willmer 2018). The colours reported in non-SDSS systems are transformed to the SDSS system following Jordi et al. (2006). The open circles with letters at their centres represent main-belt asteroids of specific taxonomic types. The objects with linear reflectivity gradients in the g − i spectral interval form the reddening line, drawn as the dashed grey curve.