Colour–colour diagrams for the Galactic Caps subset of the GSPC (|b|> 50°), with colours obtained by mixing magnitudes from the JKC and SDSS photometric systems. To highlight the stellar loci more clearly, we included only stars with |C^⋆|< 0.05 (5 968 495 of the 6 266 882 GSPC sources in the Galactic Caps sample). Upper panel: overall colour–colour diagram. Lower panel: zoom into the highly structured blue region hosting sequences of DA and non-DA WDs, hot subdwarfs and extreme horizontal branch stars, blue horizontal branch stars, and so on, better demonstrating the level of detail emerging in these diagrams.