CMDs of field NW4 (top-left), NW5 (top-right), NW6 (bottom-left), and NW7 (bottom-right). Isochrones from Marigo et al. (2008) are overlaid on the diagrams. Blue and red tracks in the first three panels indicate a metallicity of [M/H]  = −0.7 and ages of 250 Myr, 1 Gyr, and 10 Gyr. The same metallicity and age range is shown in the diagrams to mark the change in the RGB shape from NW4 to NW6. Isochrones in field NW7 (bottom-right) correspond to an age of 10 Gyr and a metallicity of [M/H]  = −0.7, −1, −1.7 (from right to left) and give an indication of the expected metallicity range in the RGB stellar population.