  This paper advances a general proposal for testing non-standard cosmological
models by means of observational relations of cosmological point sources in
some specific waveband, and their use in the context of the data provided by
the galaxy redshift surveys, but for any cosmological metric. By starting from
the general theory for observations in relativistic cosmology the equations for
colour, K-correction, and number counts of cosmological point sources are
discussed in the context of curved spacetimes. The number counts equation is
also written in terms of the selection and luminosity functions, which provides
a relativistic generalization of its Euclidean version. Since these observables
were not derived in the framework of any specific cosmology, they are valid for
any cosmological model. The hypotheses used in such derivation are reviewed,
together with some difficulties for the practical use of those observables.
