We review the phenomena having an impact on the interaction between light, paper and color halftones. They comprise surface reflections and refractions at the air-paper interface, propagation of light within the paper, internal reflections at the paper-air interface, as well as ink spreading and trapping. We introduce basic notions from radiometry and optics, such as the definitions of radiant flux, irradiance, radiance, Lambert reflector, as well as collimated and diffuse light reflection and refraction. We then present classical spectral reflection models, their extension to multi-ink color halftones and the impact of different measuring devices (radiant detector, integrated sphere). Finally we briefly review ink spreading models and highlight the topics deserving further research.