Iridescent colours are responsible for some of the most brilliant and diverse colours in the animal kingdom. Paradoxically, it remains one of the least understood mechanisms responsible for bird colouration. Unlike pigment-based colouration, where colour is produced by absorption of light, iridescent colouration is created by scattering of light by photonic structures where pigment organelles called melanosomes are the building blocks. Up to now, research has focussed on how differences in melanosome properties (e.g. dimensions, shapes, etc.) influence colour production. However, the genetic basis of iridescent colouration, its function, and the evolution of these brilliant colours remain largely unknown. Here, I propose an integrative approach linking genotype, phenotype and the evolution of iridescent colours. To do so, I first propose to identify in which taxa iridescence evolved and characterize their melanosomes. Using this data, I will reconstruct the evolution of iridescence and its functional basis (i.e. melanosome morphology) and combine it with behavioural, biological and ecological data to identify a potential function. Next, I will use state-of-the-art genomic and transcriptomic analyses to identify candidate genes responsible for iridescent colouration. Finally, I will reconstruct the evolutionary trajectories of genes responsible for iridescent colouration, identifying signatures of selection and expression shifts, as well as convergent mutations.