I won't pretend to be an expert on the cultural aspects of this, but the most compelling historical proof they have of their thesis is that chart showing the measurement of hue over a whole bunch of objects in museums, by era.

Is it possible this is a bit of... https://xkcd.com/1138/ ? The Y axis is 100% because you can only look at the objects we have, but that doesn't reflect the fact we don't have 100% of objects from 1800. We only have the objects we cared enough about to protect.

So... in someways, (in no way proof of anything) this could show the opposite? We produce a lot of junky monochrome things that get thrown away fast, and things that we care enough to protect for generations tend to be coloured. We're sort of seeing the half-life of things by colour in that chart.