  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.
