  We have developed a method to calculate overdensities in multicolour surveys,
facilitating a direct comparison of the local density contrast measured using
galaxy samples that have different redshift error distributions, i.e. for red
and blue, or bright and faint galaxies, respectively. We calculate
overdensities in small redshift slices (Delta z =0.02, which at z=0.3
corresponds roughly to Delta r_comoving=53 h^-1 Mpc) for 9176 galaxies with
R<=23.65, M_B(Vega)-5log h<=-18, and z<= 0.7, in three COMBO-17 fields
(measuring 31'X 31' each). The mean redshift errors of this sample are
approximately sigma_z/(1+z)~0.015. In the Chandra Deep Field South we identify
a region that is underdense by almost a factor 2 compared to the other two
fields in the same redshift range (0.25<~ z <~0.4). This can be used for an
investigation of the variation of the colour-dependent luminosity function with
environment: We calculate the luminosity function in this redshift range for
red sequence and blue cloud galaxies (as defined by Bell et al. 04) in each of
the fields separately. While the luminosity function of the blue galaxies
remains unaffected by different density contrasts, the luminosity function of
the red galaxies clearly has a more positive faint-end slope in the Chandra
Deep Field South as compared to the other two COMBO-17 fields. The underdensity
there is thus mainly due to a deficiency of faint red galaxies. This result is
in qualitative agreement with the trends seen at z=0.1, e.g. in the 2dFGRS
(Croton et al. 05), or in the SDSS (Zandivarez et al. 06).
