  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.
