  A large repulsion between particles in a quantum system can lead to their
localization, as it happens for the electrons in Mott insulating materials.
This paradigm has recently branched out into a new quantum state, the
orbital-selective Mott insulator, where electrons in some orbitals are
predicted to localize, while others remain itinerant. We provide a direct
experimental realization of this phenomenon, that we extend to a more general
flavour-selective localization. By using an atom-based quantum simulator, we
engineer SU(3) Fermi-Hubbard models breaking their symmetry via a tunable
coupling between flavours, observing an enhancement of localization and the
emergence of flavour-dependent correlations. Our realization of
flavour-selective Mott physics opens the path to the quantum simulation of
multicomponent materials, from superconductors to topological insulators.
