  Radial trends of stellar populations in galaxies provide a valuable tool to
understand the mechanisms of galaxy growth. In this paper, we present the first
comprehensive analysis of optical-optical and optical-NIR colours, as a
function of galaxy mass, out to the halo region (8Re) of early-type galaxies
(ETGs). We select a sample of 674 massive ETGs (M*>3x10^10MSun) from the
SDSS-based SPIDER survey. By comparing with a large range of population
synthesis models, we derive robust constraints on the radial trends in age and
metallicity. Metallicity is unambiguously found to decrease outwards, with a
measurable steepening of the slope in the outer regions (Re<R<8Re). The
gradients in stellar age are found to be more sensitive to the models used, but
in general, the outer regions of ETGs feature older populations compared to the
cores. This trend is strongest for the most massive galaxies in our sample
(M*>10^11MSun). Furthermore, when segregating with respect to large scale
environment, the age gradient is more significant in ETGs residing in higher
density regions. These results shed light on the processes leading from the
formation of the central core to the growth of the stellar envelope of massive
galaxies. The fact that the populations in the outer regions are older and more
metal-poor than in the core suggests a process whereby the envelope of massive
galaxies is made up of accreted small satellites (i.e. minor mergers) whose
stars were born during the first stages of galaxy formation.
