Mean metallicity distribution for five of the most significant merger debris in E − L_z coordinates. Different HESTA simulations are shown in different columns. The merger accretion time (Gyr), total stellar mass of the merger debris at the time of the merger (log₁₀(M_*/M_⊙)), and the stellar mass ratio (μ_*) relative to the main M31/MW progenitor at the time of the merger are given in the bottom of each panel. The mean metallicity of debris associated with the given merger, together with its standard deviation, are shown in each panel. In most cases, individual merger debris show [Fe/H] gradients, where the most metal-rich stars, formed in the centers of accreting galaxies, have the lowest energies. Meanwhile, low-[Fe/H] stars formed in the outer regions of dwarf galaxies accreted first and, thus, have higher energy inside the host M31/MW galaxies.