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Among the properties shaping the light of a galaxy, the star formationhistory (SFH) is one of the most challenging to model due to the variety ofcorrelated physical processes regulating star formation. In this work, weleverage the stellar population synthesis model FSPS, together with SFHspredicted by the hydrodynamical simulation IllustrisTNG and the empirical modelUNIVERSEMACHINE, to study the impact of star formation variability on galaxycolours. We start by introducing a model-independent metric to quantify theburstiness of a galaxy formation model, and we use this metric to demonstratethat UNIVERSEMACHINE predicts SFHs with more burstiness relative toIllustrisTNG. Using this metric and principal component analysis, we constructfamilies of SFH models with adjustable variability, and we show that theprecision of broad-band optical and near-infrared colours degrades as the levelof unresolved short-term variability increases. We use the same technique todemonstrate that variability in metallicity and dust attenuation presents apractically negligible impact on colours relative to star formationvariability. We additionally provide a model-independent fitting functioncapturing how the level of unresolved star formation variability translatesinto imprecision in predictions for galaxy colours; our fitting function can beused to determine the minimal SFH model that reproduces colours with sometarget precision. Finally, we show that modelling the colours of individualgalaxies with percent-level precision demands resorting to complex SFH models,while producing precise colours for galaxy populations can be achieved usingmodels with just a few degrees of freedom.

Surrogate modelling the Baryonic Universe II: on forward modelling the colours of individual and populations of galaxies