  We investigate the evolution of oxygen- and carbon-rich AGB stars, post-AGB
objects, and planetary nebulae using data collected mainly from the MSX
catalogue. Magnitudes and colour indices are compared with those calculated
from a grid of synthetic spectra that describe the post-AGB evolution beginning
at the onset of the superwind. We find that carbon stars and OH/IR objects form
two distinct sequences in the (K-[8.3])x([8.3]-[14.7]) MSX colour diagram.
OH/IR objects are distributed in two groups: the bluest ones are crowded near
[14.7]-[21.3]=1 and [8.3]-[14.7]=2, and a second, redder group is spread over a
large area in the diagram, where post-AGB objects and planetary nebulae are
also found. High mass-loss rate OH/IR objects, post-AGB stars, and planetary
nebulae share the same region in the (K-[8.3])x([8.3]-[14.7]) and
[14.7]-[21.3]x([8.3]-[14.7]) colour-colour diagrams. This region in the diagram
is clearly separated from a bluer one where most OH/IR stars are found. We use
a grid of models of post-AGB evolution, which are compared with the data. The
gap in the colour-colour diagrams is interpreted as the result of the rapid
trajectory in the diagram of the stars that have just left the AGB.
