Period-color (PC) relations may be used to study the interaction of the
stellar photosphere and the hydrogen ionization front (HIF). RR Lyraes
(RRLs) and long period classical Cepheids (P > 10d) have been found
to exhibit different PC behavior at minimum and maximum light which can
be explained by the HIF-photosphere interaction based on their location
on the HR diagram. In this work, we extend the study to include type II
Cepheids (T2Cs) with an aim to test the HIF-photosphere interaction
theory across a broad spectrum of variable star types. We find W Vir
stars and BL Her stars to have similar PC relations as those from long
period and short period classical Cepheids, respectively. We also use
MESA to compute RRL, BL Her and classical Cepheid models to study the
theoretical HIF-photosphere distance and find the results to be fairly
consistent with the HIF-photosphere interaction theory.