Variability of late-type stars on timescales of days or longer is largely due to magnetic surface features (starspots and faculae). We present ACTReSS, a software tool to calculate rotational lightcurves of stars. The model uses limb-dependent intensities derived from MURaM 3-D magneto-convection simulations (Beeck et al. 2014) for quiet stellar photospheres and bright active regions. This allows us to investigate the dependence of the flux variation on spectral type, inclination and wavelength.