Unlike the Dome of the Rock, which reflects classical Byzantine architecture, the Al-Aqsa Mosque is characteristic of early Islamic architecture. Nothing remains of the original dome built by Abd al-Malik. The present-day dome was built by az-Zahir and consists of wood plated with lead enamelwork. In 1969, the dome was reconstructed in concrete and covered with anodized aluminum, instead of the original ribbed lead enamel work sheeting. In 1983, the aluminum outer covering was replaced with lead to match the original design by az-Zahir.
So it never was silver to begin with. I’ve heard somewhere (National Geographic?) that the gold dome is anodized aluminum, too, and judging from old photographs, both coverings of anodized aluminum were added by the Jordanians, around 1960. Personally, I don’t think the lead looks as good.