The colour G − H_p as a functionof the BP − RP colour. Colours computed from models of different metallicity are shown with a different colour: black for [M/H] = 0.0, red for [M/H] = − 1.0, green for [M/H] = − 2.0 and blue for [M/H] = − 3.0. Open circles are the colours computed from ATLAS models, while filled squares are the corresponding 3D-corrected colours. We stress that the Gaia bands are the pre-launch bands.