Recent experiments with Tetenal Gold Toner, a precious commodity nowadays. One litre costs nearly £100, if you can find anyone that still has stock of it.
Counterintuitively, gold toner imparts a bluish tone. It is the most archival post-processing technique you can use, I will have deteriorated long before the image ever does! You can see the difference between the original colour (right) and the toned image (left) here:
I’ll write more about this on my site at Real Photographs.