  Through matches with the Sloan Digital Sky Survey (SDSS) catalogue we
identify the location of various families of astronomical objects in WISE
colour space. We identify reliable indicators that separate Galactic/local from
extragalactic sources and concentrate here on the objects in our Galaxy and its
closest satellites. We develop colour and magnitude criteria that are based
only on WISE data to select asymptotic giant branch (AGB) stars with
circumstellar dust shells, and separate them into O-rich and C-rich classes.
With these criteria we produce an all-sky map for the count ratio of the two
populations. The map reveals differences between the Galactic disc, the
Magellanic Clouds and the Sgr Dwarf Spheroidal galaxy, as well as a radial
gradient in the Large Magellanic Cloud (LMC) disc. We find that the C:O number
ratio for dusty AGB stars increases with distance from the LMC centre about
twice as fast as measured for near-IR selected samples of early AGB stars.
Detailed radiative transfer models show that WISE colours are well explained by
the emission of centrally heated dusty shells where the dust has standard
properties of interstellar medium (ISM) grains. The segregation of different
classes of objects in WISE colour space arises from differences in properties
of the dust shells: those around young stellar objects have uniform density
distributions while in evolved stars they have steep radial profiles.
