Colours of stars and galaxies hold information on intrinsic and extrinsic parameters, such as stellar temperatures and galaxy populations, selective absorption and colour changes due different dust contents and inclination angles of galaxies, as well as galaxy redshifts and evolution.
The colour-magnitude diagram of field stars comprises objects at largely different distances, representing two or three different stellar populations - halo, thin disk and possibly thick disk. With the aid of astrometry, we can interpret colour magnitude diagrams for stars with measured proper motions µ > 100mas/yr. This helped us to find good starting parameters for simulations used to explain the colour-magnitude diagrams of all stars. Preliminary results support the presence of a thick disk.
The colour-magnitude diagram of galaxies is a mixture, predominantly of contributions from morphological types, including dust and dust distribution, aspects, redshifts z, evolutionary processes and K-corrections. The colour magnitude diagram of 65000 galaxies to which all above-mentioned quantities and others contribute can only be explained by comparison with simulations.
Tracings the effects of galaxy evolution for magnitudes r_F 20 with large statistical samples may lend credulity to the conclusions drawn from the much smaller samples available until now. The comparison of colour distributions of intrinsically faint and bright galaxies may yield constraints for the time of formation of massive and low mass galaxies.
Colours are also if interest for objects which we identify as quasar candidates on the basis of our objective prism spectra. Almost 12000 candidates have been found so far. From comparison with known redshifts and redshift from objective prism plates, we expect to find usable redshifts for about 75 % of the candidates. In spite of a bias towards Lyman alpha quasars, the colour distribution of this sample will be of interest by comparison with quasar surveys where the objects are selected by colour.
Colours are also of technical help. Together with the morphological types of galaxies, obtained automatically within the MRSP up to b_J = 19.5, information is obtained about the mixture of galaxy types in samples and thus on the K -corrections to be applied. This avoids systemativ errors due to global K-corrections which may lead to systematic errors of luminosities and distances.
Finally, one might mention the possibilities of checking cosmological findings obtained by using b_J magnitudes. Controversial conclusions, such as the ones drawn from the galaxy counts N(b_J) within the APM Project may be independently supported or rejected with N(r_F) counts. For large samples of galaxies angular correlations functions may be determined on the basis of their r_F magnitudes in addition to their b_J magnitudes, again for comparison with earlier results, which were obtained from essentially the same poool of objects by the same techniques, except that there may be different selection criteria and different measuring errors.
For fluctuation analysis in cosmology, using three-dimensional galaxy counts-in-cells, magnitude measurements of the same objects in different colour ranges may help to smooth the effects of magnitude errors, which must be small when larger scales are to be reached. Large-scale density fluctuation analysis needs large survey volumes of equal extent in all spatial dimensions and is thus a task which can be achieved by photographic means with their large sky coverage, and with z -values from objective prism plates if the magnitudes can thus be sufficiently improved.