Colour evolution of SN 2018oh compared to models with and without ⁵⁶Ni shells in the outer ejecta. Observations of SN 2018oh are shown as black points. Our fiducial model is shown as a solid grey line. Models with additional ⁵⁶Ni shells are shown as coloured lines and our model with an extended ⁵⁶Ni distribution relative to our fiducial model is shown as a grey dashed line. Based on comparisons to our models, we infer an explosion date of MJD = 58144.1, which is shown as a vertical dash-dot line. The epoch of our spectral comparison is shown as a vertical dotted line.