From rest-frame luminosity functions to observer-frame colour
  distributions: tackling the next challenge in cosmological simulations
  Galaxy spectral energy distributions (SEDs) remain among the most challenging
yet informative quantities to reproduce in simulations due to the large and
complex mixture of physical processes that shape the radiation output of a
galaxy. With the increasing number of surveys utilising broadband colours as
part of their target selection criteria, the production of realistic SEDs in
simulations is necessary for assisting in survey design and interpretation of
observations. The recent success in reproducing the observed luminosity
functions (LF) from far-UV to far-IR, using the state-of-the-art semi-analytic
model \shark\ and the SED generator \prospect, represents a critical step
towards better galaxy colour predictions. We show that with \shark\ and
\prospect\ we can closely reproduce the optical colour distributions observed
in the panchromatic GAMA survey. The treatment of feedback, star formation,
central-satellite interactions and radiation re-processing by dust are critical
for this achievement. The first three processes create a bimodal distribution,
while dust attenuation defines the location and shape of the blue and red
populations. While a naive comparison between observation and simulations
displays the known issue of over-quenching of satellite galaxies, the
introduction of empirically-motivated observational errors and classification
from the same group finder used in GAMA greatly reduces this tension. The
introduction of random re-assignment of $\sim 15\%$ of centrals/satellites as
satellites/centrals on the simulation classification closely resembles the
outcome of the group finder, providing a computationally less intensive method
to compare simulations with observations.
