  We present an analysis of the effects of environment on the photometric
properties of galaxies in the core of the Shapley Supercluster at z=0.05, one
of the most massive structures in the local universe. The Shapley Optical
Survey (SOS) comprises archive WFI optical imaging of a 2.0 deg^2 region
containing the rich clusters A3556, A3558 and A3562 which demonstrate a highly
complex dynamical situation including ongoing cluster mergers. The B-R/R
colour-magnitude relation has an intrinsic dispersion of 0.045 mag and is
0.015\pm0.005 mag redder in the highest-density regions, indicative of the red
sequence galaxy population being 500 Myr older in the cluster cores than
towards the virial radius. The B-R colours of galaxies are dependent on their
environment, whereas their luminosities are independent of the local density,
except for the very brightest galaxies (M_R<-22). The global colours of faint
(>M*+2) galaxies change from the cluster cores where ~90% of galaxies lie along
the cluster red sequence to the virial radius, where the fraction has dropped
to just ~20%. This suggests that processes related to the supercluster
environment are responsible for transforming faint galaxies, rather than galaxy
merging, which should be infrequent in any of the regions studied here. The
largest concentrations of faint blue galaxies are found between the clusters,
coincident with regions containing high fractions of ~L* galaxies with radio
emission indicating starbursts. Their location suggests star-formation
triggered by cluster mergers, in particular the merger of A3562 and the poor
cluster SC1329-313, although they may also represent recent arrivals in the
supercluster core complex. (abstract truncated)
