Great suggestion. I made the change now! Please have a look: https://www.metallicss.com
Would love to see a write-up of how you made this work. I took a peak at the repo and I can see it's using a bunch of SVG filters, which I think is underutilized piece of tech!
Some feedback on the effect: I think the copper and the gold could use a bit more tweaking. The gold has a bit too much green, and the copper has a bit too much red perhaps? It looks kind of earth coloured.