This is a very interesting perspective. I'd thought the muted, brownish colors in these paintings had to do with the quality and availability of pigments during that period.

There's most likely multiple aspects at play: high-chroma pigments were historically limited and/or expensive; varnish yellowed over time; pigments faded. The digitization process probably wasn't perfect as well (I'd expect modern scans should be fairly good though).

hundreds of years of oxidation will make everything brown.