Was out of town over the weekend, and had the chance to spend an hour in a secondhand English-language bookshop. Picked up a bunch of books, including this one -- a 50+ year old paperback on painting. (Because, you see, I know very little about painting. I took an art history course years ago, and I guess I have a layman's knowledge of the major schools, but that's about it.)
Anyway! This book, out of print for fifty years, turned out to be a little gem. It's full of all sorts of good discussion, pitched right about at my level -- to the reader who knows who Goya and Giotto are, but who doesn't paint and knows almost nothing about the actualy craft of painting.
Here's one thing I discovered: traditional artists studios faced north, because the artists did not want to paint with different slants of light coming in the window at different times and seasons. Makes perfect sense, right?
But -- northern light is blue. So painters would cover the walls of their studios with red and orange hangings and tapestries, in order to warm the light. So apparently (I have yet to check this), in many paintings up until the middle of the 19th century, the light is somewhat blue but the shadows are somewhat reddish or brownish.
Further. Up until around 1840, the vast majority of paintings were done indoors, usually in those studios. Why? Because portable oil paints didn't exist yet. Paints were still mixed by hand. Almost every painter had to be a paintmaker as well, and had to start with lumps and powders and turn them into paints shortly before beginning to work. (The exception was watercolors, which is why most outdoor scenes before then were watercolors.) So even when a painter was doing an outdoor scene, he'd often be working from memory, in his studio. Apparently some painters would go so far as to take rocks or other outdoor items into the studio in order to have them convenient as models.
But around 1840, the first containers of portable oil paint appeared -- the squeezable tubes that we think of today when we think of artists' paint. And one consequence of that was an explosion of outdoor painting. And outdoors, the light is often yellow or orange, while the shadows are likely to be bluish. Apparently this took a lot of getting used to. But it's one of the things that, a bit later, would distinguish the Impressionists: not only were they doing weird stuff with their brushstrokes, but the colors were all different.