Dust de-reddened J-PLUS colour–colour diagrams of MS (left panels) and GB stars (right panels). From top to bottom: (u − g)₀ vs. (g − r)₀; (J0430 − J0861)₀ vs. (u − J0378)₀; (J0395 − J0515)₀ vs. (r − J0660)₀. The colour scale shows the density of sources, increasing from blue to red. The J-PLUS colours from the synthetic photometry of the Pickles (1998) empirical library are shown as grey symbols (squares for luminosity class V and triangles for luminosity class III). The location of low-metallicity halo stars and blue horizontal branch (BHB) stars is marked with a black line in the upper and middle left panels.