Gaia DR2 effective temperatures and BP − RP colours of old (red filled lozenges) and new (open blue lozenges) Ap stars. The overplotted black open squares and dots mark the most distant and astrometrically problematic objects (Sect. 2.1), respectively. In the colour range 0.2 <  BP − RP <  0.6 (between the dashed lines), new Ap stars show a larger scatter and systematically hotter T_eff. The slope of the approximately linear relationship becomes flatter at BP − RP >  0.6.