Overview of the target stars in multiple colour-colour spaces. The data were gathered from multiple sources: the Panoramic Survey Telescope and Rapid Response System 1 3Π survey Data Release 2 (Chambers et al. 2016; Magnier et al. 2020); the Canada-France Imaging Survey; the Sloan Digital Sky Survey Data Release 18 (Almeida et al. 2023); and the Pristine Survey (Starkenburg et al. 2017). The candidate BHB stars in our sample are colour-coded according to the ID number given in Table 1. The background stars come from two samples. The pink contours are blue straggler stars from the Xue et al. (2008) set while the brown contours represent BHB stars from the Xue et al. (2011) dataset (see the markers of the same colours in the upper left and upper right corners in the top and lower panels, respectively, for the average uncertainties of these stars). We use different markers to indicate the source of each star (Thomas et al. (2018), Starkenburg et al. (2019), or both catalogues). We decided to not show the stars with red colour indices (u_SDSS-Pristine CaHK)₀ in the lower middle panel that came from the catalogue Thomas et al. (2018) and just represent them with triangles pointing to the right.