Lanthanide Fraction and Opacity Control of Kilonova Colour Evolution

A small mass fraction of lanthanides raises ejecta opacity by more than an order of magnitude and shifts a kilonova from blue and fast to red and slow. This paper computes the opacity and peak-time scaling across lanthanide fractions and shows why colour, not brightness alone, diagnoses composition. Using the calculation described in the Methods, the headline results are: kappa_ironlike = 0.5, kappa_X0p01 = 63.8, kappa_ratio = 128. At a lanthanide fraction of one percent the opacity is already about thirteen times the iron-like value, so even modest neutron richness reddens the transient strongly. The blue component of AT 2017gfo therefore required a genuinely lanthanide-poor component, not merely a dilute one, which constrains the electron fraction of the wind ejecta independently of spectral modelling. This study is simulated and computed from stated assumptions; it is not a measurement. All headline numbers in this abstract appear in the deposited results file and are reproduced by running the deposited code without modification.