The hydrodynamics of the outer envelope structure in classical pulsators can be studied using the period-colour relations at different pulsation phases. In this work, the hydrogen ionization front (HIF)-stellar photosphere interaction theory is probed from both the observational and theoretical aspects. Empirical period-colour (PC) relations for the classical pulsators exhibit flat/sloped PC relations for RRab stars at minimum/maximum light; sloped/flat PC results for W~Vir stars and long-period (P$>$10d) classical Cepheids at min/max light and intermediate behaviour with a much smaller difference between the PC slopes at max/min light for BL~Her stars and short-period (P$<$10d) Cepheids. The theoretical aspect involves studying the HIF-stellar photosphere distances from stellar pulsation models computed using MESA-RSP. We found the results from the models to be generally consistent with this picture and thereby demonstrated that the theory holds true across a broad spectrum of variable stars in different evolutionary stages.