We describe Period-Color and Amplitude-Color Relations in Cepheids and RR Lyraes and describe how such relations
are consistent with considerations of the interaction of the hydrogen ionization front and stellar photosphere. We
show how such an interaction can provide a physical explanation of the observed non-linearity of the Cepheid PL
relation including possible impacts on CMB independent estimates of Hubble's constant. We close with a discussion
of how multiphase PL/PC/PW relations can provide insight into the observed characteristics of the mean light
relations.
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