  We explore the behavior of the blue galaxy fraction over the redshift range
0.75 <= z <= 1.3 in the DEEP2 Survey, both for field galaxies and for galaxies
in groups. The primary aim is to determine the role that groups play in driving
the evolution of galaxy colour at high z. The colour segregation observed
between local group and field samples is already in place at z ~ 1: DEEP2
groups have a significantly lower blue fraction than the field. At fixed z,
there is also a correlation between blue fraction and galaxy magnitude, such
that brighter galaxies are more likely to be red, both in groups and in the
field. In addition, there is a negative correlation between blue fraction and
group richness. In terms of evolution, the blue fraction in groups and the
field remains roughly constant from z=0.75 to z ~ 1, but beyond this redshift
the blue fraction in groups rises rapidly with z, and the group and field blue
fractions become indistinguishable at z ~ 1.3. Careful tests indicate that this
effect does not arise from known systematic or selection effects. To further
ensure the robustness of this result, we build on previous mock DEEP2
catalogues to develop mock catalogues that reproduce the colour-overdensity
relation observed in DEEP2 and use these to test our methods. The convergence
between the group and field blue fractions at z ~ 1.3 implies that DEEP2 galaxy
groups only became efficient at quenching star formation at z ~ 2; this result
is broadly consistent with other recent observations and with current models of
galaxy evolution and hierarchical structure growth. (Abridged.)
