CaHK colour–colour diagram, with the temperature proxy (g − i)₀ on the x axis and the Pristine colour containing the metallicity-sensitive information. The density map in the background of the plot is produced using MW stars in the field of Boo II, with 1, 2 and 3σ contours shown as dotted white lines. In this diagram, the metallicity goes down as the y axis colour decreases. The dashed straight line shows the cut applied to the data to distinguish likely the metal-poor from the likely metal-rich stars. The colours and markers used are the same as those of Fig. 1.