Fit of multiband (UBVgri) early light curves of SN 2018xx (left panel) and SN 2019gbx (right panel). The dashed lines show the expanding fireball model (i.e., f ∝ (t − FLT)²) (Riess et al. 1999) fitting by forcing all six bands to have the same FLT and using the data with t < −7 days. The bottom subpanel in each image displays the residual of the best-fit curves, and the gray dashed horizontal line represents zero residual.