Earliest simultaneous multicolour optical observations of GRB 230328B: from 41 s to the host-galaxy identification

We present a multiwavelength study of the long-duration gamma-ray burst GRB 230328B, combining prompt (Formula presented) -ray observations with exceptionally early simultaneous optical photometry and extensive X-ray, radio, and host-galaxy follow-up. NUTTELA-TAO began simultaneous (Formula presented) observations only 41 s after the Swift/BAT trigger, corresponding to approximately 16 s in the rest frame for the adopted host-galaxy photometric redshift of (Formula presented). Unlike sequential multiband measurements, these observations provide an instantaneous optical spectral slope without temporal-interpolation uncertainties. The early optical continuum remains statistically consistent with a constant slope from approximately (Formula presented) to (Formula presented) s, providing no evidence for rapid dust destruction. The absence of significant colour evolution across the pronounced optical rebrightening at approximately (Formula presented) s demonstrates that it is achromatic and favours a dynamical rather than spectral origin. Broad-band modelling shows that the optical, X-ray, and radio evolution can be broadly explained by forward-shock emission with late energy injection, although alternative scenarios cannot be excluded. The red optical continuum is consistent with substantial line-of-sight extinction in a massive, dusty host galaxy. Late-time observations are insufficient to constrain a typical GRB-associated supernova at the adopted redshift. GRB 230328B therefore provides a benchmark for connecting the earliest simultaneous optical colours with afterglow dynamics and the host environment of a representative long GRB.