Another excellent question to which I don't have an excellent answer, @cloudspyder! We were wondering the same thing. Close up, it looks like gold paint...which I guess is gold, isn't it?

The dome of the Invalides is definitely covered in gold leaf (not paint) - quite extraordinary. On our bus tour they told us how much the gold leaf weighs, but we didn't have our notebooks out so I'd have to look that one up again. The number is BIG.

Thanks for stopping in! 😊😊