CRASH2: coloured packets and other updates

In this paper, we report on the improvements implemented in the cosmological radiative transfer code CRASH. In particular, we present a new multi-frequency algorithm for spectra sampling which makes use of coloured photon packets: we discuss the need for the multi-frequency approach, describe its implementation and present the improved CRASH performance in reproducing the effects of ionizing radiation with an arbitrary spectrum. We further discuss minor changes in the code implementation which allow for more efficient performance and an increased precision