  We use photometric redshifts to analyse the effect of local environment on
galaxy colours at redshifts z < 0.63 in the SDSS data release 6. We construct
mock SDSS-DR6 catalogues using semi-analytic galaxies to study possible
systematic effects on the characterisation of environment and colour statistics
due to the uncertainty in the determination of redshifts. We use the projected
galaxy density derived from the distance to the nearest neighbours with a
suitable radial velocity threshold to take into account the uncertainties in
the photometric redshift estimates. Our findings indicate that the use of
photometric redshifts greatly improves estimates of projected local galaxy
density when galaxy spectra are not available. We find a tight relationship
between spectroscopic and photometric derived densities, both in the SDSS-DR6
data (up to z=0.3) and mock catalogues (up to z=0.63).
  At z=0, faint galaxies show a clear increase of the red galaxy fraction as
the local density increases. Bright galaxies, on the other hand, show a
constant red galaxy fraction. We are able to track the evolution of this
fraction to z=0.55 for galaxies brighter than M_r=-21.5 and find that the
fraction of blue galaxies with respect to the total population progressively
becomes higher as the redshift increases, at a rate of 15% Gyr. Also, at any
given redshift, bright galaxies show a larger red population, indicating that
the star-formation activity shifts towards smaller objects as the redshift
decreases.
