Cesca chairs authenticity  

I found 6 of these chairs and was happy I might own the originals. First strange thing that the tubes turned yellow or goldish. As I don't know metals I have no idea if such thing can happen to chrome over time.
The owner said he bought them in a large german furniture store chain that since then became the KIKA chain - in the 90's.
It was just strange to believe that a large company would sell fakes. But could they be authentic if I don't see any signs or marks on them?
Thanks for any opinions.