  A parameter study of the spectral evolution of a typical post-AGB star, with
particular emphasis on the evolution of the infrared colours, is presented. The
models are based on the latest evolutionary tracks for hydrogen burning
post-AGB stars. For such tracks the evolutionary rate is very dependent on the
assumed mass loss rate as a function of time. We investigate this effect by
modifying the mass loss prescription. The newly calculated evolutionary rates
and density distributions are used to model the spectral evolution of a
post-AGB star with the photo-ionization code Cloudy, including dust in the
radiative transfer. Different assumptions for the dust properties and dust
formation are considered. It is shown that by varying these parameters in a
reasonable way, entirely different paths are followed in the IRAS colour-colour
diagram. First of all, the effects of the evolution of the central star on the
expanding dust shell can not be neglected. Also the dust properties and the
definition of the end of the AGB phase have an important effect. The model
tracks show that objects occupying the same location in the IRAS colour-colour
diagram can have a different evolutionary past, and therefore the position in
the IRAS colour-colour diagram alone can not a priori give a unique
determination of the evolutionary status of an object. An alternative
colour-colour diagram, the K-[12] vs. [12]-[25] diagram, is presented. The
tracks in this diagram seem less affected by particulars of the grain emission.
This diagram may be a valuable additional tool for studying post-AGB evolution.
