Training sample (CaMD cut on good enough parallax giants and bad parallax giants selected after the magnitude cut) colour-coded by log g and the colour-metallicity cut in Equation (2), shown as a black line. Any stars that lie under this line are removed. The cut is designed to remove mainly subgiant stars probed unevenly at different metallicity ranges, and the colour-coding shows that most stars underneath this line has log g roughly greater than 3.5 (not a giant). The cut preserves the RGB (the blue region) and cuts away the SG/MSTO (the red region).