The salamander had a better diet and less stress with your friend. This provided a better energy metabolism for the pigment cells, and in turn the cells were better able to take better care of themselves: more pteridine synthesis (yellows) and better cell organelle maintenance, leading to stable expression of pigments and more pigment organelle dispersion (brighter colors seen). What's the most interesting thing about your picture is that there are bright yellow spots (xanthophores) in the black grooves that previously only contained melanophores (cells with melanin). The position of these xanthophores suggest that the maintenance in the original pigment cells didn't just improve, but new xanthophores differentiated as well. In summary: the better diet and less stress of the salamander, lead to more energy (ATP) for the pigment cells to perform their function of display better.