Isochrones from Dartmouth at age 11 Gyr and metallicities ranging from −1.5 to +0.5 dex (top) and along the age-metallicity relation of Fig. 3 (bottom), in the theoretical CMD in different WFC3 colors. The horizontal short segment in each CMD is 0.2 mag long, to be compared with the width of the turn-off in each color band. The color separation at turn-off between the most metal-rich and metal-poor isochrones is 0.21 mag (top) and 0.074 mag (bottom) in the F606W−F814W, similarly to Figs. 5 and 3. The color separation at the turn-off is 0.333 and 0.124 mag in the F555W−F814W bands. Alpha abundances of the isochrones are the same as those used for the isochrones of Figs. 5 and 3.