  It is possible to reliably identify white dwarfs (WDs) without recourse to
spectra, instead using photometric and astrometric measurements to distinguish
them from Main Sequence stars and quasars. WDs' colours can also be used to
infer their intrinsic properties (effective temperature, surface gravity,
etc.), but the results obtained must be interpreted with care. The difficulties
stem from the existence of a solid angle degeneracy, as revealed by a full
exploration of the likelihood, although this can be masked if a simple best-fit
approach is used. Conversely, this degeneracy can be broken if a Bayesian
approach is adopted, as it is then possible to utilise the prior information on
the surface gravities of WDs implied by spectroscopic fitting. The benefits of
such an approach are particularly strong when applied to outliers, such as the
candidate halo and ultra-cool WDs identified by Vidrih et al. (2007). A
reanalysis of these samples confirms their results for the latter sample but
suggests that that most of the halo candidates are thick disk WDs in the tails
of the photometric noise distribution.
