In this paper , we construct full amplitude non-linear hydrodynamical models of fundamental mode Galactic Cepheids and analyze the resulting theoretical period-colour and amplitude-colour relations at maximum , mean and minimum light .
These theoretical relations match the general form of the observed relations well .
This agreement is , to some extent , independent of the mass-luminosity relations used , pulsation code , numerical techniques , details of the input physics and methods to convert theoretical quantities , such as bolometric luminosity and temperature , to observational quantities , such as V band magnitudes or ( V - I ) colours .
We show that the period-colour and amplitude-colour properties of fundamental mode Galactic Cepheids with periods such that \log ( P ) > 0.8 can be explained by a simple application of the Stefan-Boltzmann law and the interaction of the photosphere with the hydrogen ionization front .
We discuss the implications of our results for explaining the behavior of Galactic Cepheid period-colour , and period-luminosity relations at mean light .