  By combining optical and near-IR observations from the Hubble Space Telescope
with NIR photometry from the Spitzer Space Telescope it is possible to measure
the rest-frame UV-optical colours of galaxies at z=4-8. The UV-optical spectral
energy distribution of star formation dominated galaxies is the result of
several different factors. These include the joint distribution of stellar
masses, ages, and metallicities, and the subsequent reprocessing by dust and
gas in the ISM. Using a large cosmological hydrodynamical simulation we
investigate the predicted spectral energy distributions of galaxies at
high-redshift with a particular emphasis on assessing the potential
contribution of nebular emission. We find that the average pure stellar
UV-optical colour correlates with both luminosity and redshift such that
galaxies at lower-redshift and higher-luminosity are typically redder. Assuming
the escape fraction of ionising photons is close to zero, the effect of nebular
emission is to redden the UV-optical 1500-V_w colour by, on average, 0.4 mag at
z=8 declining to 0.25 mag at z=4. Young and low-metallicity stellar
populations, which typically have bluer pure stellar UV-optical colours,
produce larger ionising luminosities and are thus more strongly affected by the
reddening effects of nebular emission. This causes the distribution of 1500-V_w
colours to narrow and the trends with luminosity and redshift to weaken. The
strong effect of nebular emission leaves observed-frame colours critically
sensitive to the source redshift. For example, increasing the redshift by 0.1
can result in observed frame colours changing by up to ~0.6. These predictions
reinforce the need to include nebular emission when modelling the spectral
energy distributions of galaxies at high-redshift and also highlight the
difficultly in interpreting the observed colours of individual galaxies without
precise redshifts.
