Non-uniform age selection for models with instantaneous star formation. The blue points indicate our age samples (horizontal axis), which depend on the metallicity. The vertical dotted lines indicate age values that we always sample (2 per dex). We choose them using an algorithm that picks the minimum number of points to achieve <10% interpolation error in all bands. Note that while this figure shows the results for seven metallicities available in the Padova tracks for completeness, we only use five of them in the DIRTYGrid.