The formation and growth of galaxy spheroids and their central black holes are now viewed as intimately related processes, and establishing how the mass of quasar host galaxies changes as a function of redshift provides a key measurement for constraining this process. From observations using NICMOS on HST we have derived the best estimate to date of the luminosity evolution of the hosts of both radio-quiet and radio-loud quasars from the local universe out to z=2. However, in order to accurately convert these host luminosities into galaxy masses we require additional information on the stellar composition of the hosts. New observations of these quasar hosts with HST/WFPC2 are now allowing us to derive the first reliable rest-frame U-V colours for high-z quasar hosts the results of this study are discussed here.