Mid-infrared period-color data for Cepheids in the Galaxy (open triangles), LMC (open circles), NGC 6822 (blue dots), and IC 1613 (red squares). Photometry for the remote galaxies are too imprecise to directly assess model predictions concerning the metallicity dependence (zero-point) of the period-color relation (e.g., Ngeow et al. 2012; Monson et al. 2012, the latter’s Fig. 10). For clarity purposes photometric uncertainties (probably underestimated) are provided for IC 1613 only, and the Galactic observations were shifted (ordinate) to match the LMC observations.