High-order harmonic generation (HHG) is a sensitive probe of atomic and molecular structures. This research field has greatly benefited from the exploitation of mid-IR driving sources that allowed the extension of the harmonic emission to higher photon energies, giving access to several phenomena previously unexplored with this technique, such as the giant resonance in xenon. This enhancement in the harmonic generation yield around 100 eV has been interpreted in terms of the electronic structure of xenon, suggesting the key role of single and multi-electron contributions to the harmonic generation process. We extensively studied this phenomenon experimentally using HHG by a two-color field, combining this powerful experimental approach with a mid-IR driving source, providing the evidence of a deviation of the xenon response with respect to the expected single electron behavior which is interpreted as the fingerprint of electron correlation effects.