Top: colour–magnitude diagram from Galactic simulations presenting dwarfs and subgiant stars with V < 13 and T_eff < 7220 K (i.e. later than F0). The arrow indicates the average direction of the reddening vector. Bottom: percentage of targets (continuous line) and contaminants (i.e. stars with spectral type earlier than F5, dotted line) within 0.05 mag wide colour intervals. The red dashed vertical lines denote our adopted blue colour limit of the PLATO FGK sample, the blue dotted vertical lines indicate the colour where the selection is 100% dominated by contaminants, while the blue long-dashed vertical lines show the colour where the selection is 100% dominated by the targets.