  We develop a method to compute synthetic kilonova light curves that combines
numerical relativity simulations of neutron star mergers and the
$\texttt{SNEC}$ radiation-hydrodynamics code. We describe our implementation of
initial and boundary conditions, r-process heating, and opacities for kilonova
simulations. We validate our approach by carefully checking that energy
conservation is satisfied and by comparing the $\texttt{SNEC}$ results with
those of two semi-analytic light curve models. We apply our code to the
calculation of color light curves for three binaries having different mass
ratios (equal and unequal mass) and different merger outcome (short-lived and
long-lived remnants). We study the sensitivity of our results to hydrodynamic
effects, nuclear physics uncertainties in the heating rates, and duration of
the merger simulations. We find that hydrodynamics effects are typically
negligible and that homologous expansion is a good approximation in most cases.
However, pressure forces can amplify the impact of uncertainties in the
radioactive heating rates. We also study the impact of shocks possibly launched
into the outflows by a relativistic jet. None of our models match AT2017gfo,
the kilonova in GW170817. This points to possible deficiencies in our merger
simulations and kilonova models which neglect non-LTE effects and possible
additional energy injection from the merger remnant, and to the need to go
beyond the assumption of spherical symmetry adopted in this work.
