Relevant stars along the line of sight towards the Dorado field. This example showcases the selection process conducted prior to deriving colour transformations. The coloured dots depict the simulated stellar population along the line of sight, as provided by the Besançon model of the Galaxy. In the two left panels, the colour coding represents metallicity [M/H]. In the two right panels, the colour coding corresponds to [α/Fe], with black symbols marking the models available within the data-containing range of the simulation. Grey lines delineate the approximate boundaries we established for this selection process, ensuring that the subsequent colour transformations accurately reflect the characteristics of the stellar population along the observed line of sight.