Gaia colour-colour plot of the optical counterparts associated with the eROSITA sources in the eRASS1 variability catalogue. The classification of the sources as Galactic or extragalactic follows Sect. 5.3. Most of the sources are classified as coronal stars from Freund et al. (2024), while for the remaining sources identified in Salvato et al., in prep., the classification is derived from the Gaia properties listed in the original catalogue with a probability of being galaxies, QSOs, or stars higher than 50%. Few sources are classified as star or AGN based on their WISE colours or as QSO based on the Veron & Veron catalogue. Most of the sources are stellar, and only 10% of the sources are classified as of extragalactic nature.