  We computed the luminosity of simulated galaxies of the C-EAGLE project, a
suite of 30 high-resolution zoom-in simulations of galaxy clusters based on the
EAGLE simulation. The AB magnitudes are derived for different spectral bands,
from ultraviolet to infrared, using the simple stellar population modeling
based on the E-MILES stellar spectra library. We take into account obscuration
due to dust in star forming regions and diffuse interstellar medium. The $g-r$
colour-stellar mass diagram, at z=0.1, presents a defined red sequence,
reaching $g-r \simeq 0.8$, 0.05 dex redder than EAGLE at high masses, and a
well populated blue cloud, when field galaxies are included. The clusters'
inner regions are dominated by red-sequence galaxies at all masses, although a
non-negligible amount of blue galaxies are still present. We adopt Bayesian
inference to compute the clusters LFs, testing for statistical significance of
both single and double Schechter functions. The multicolour LFs at z=0 show a
knee luminosity that peaks in the infrared and increases with the cluster's
mass. The faint-end is weakly dependent on colour and mass and shows an upturn
in the optical, bounded between -1.25 and -1.39, just moderately steeper than
the field. The simulations reproduce, within the observational errors, the
spectroscopic LFs of the Hercules and Abell 85 clusters, including their faint
end upturn. C-EAGLE LFs are in broad agreement with observed LFs taken from
SDSS and XXL surveys, up to z=0.67, showing a rather flat faint end when the
observational constrains are taken into account.
