I think this is part of it. I don't think people (in real life, not in magazines) cared as much whether something "went with" the rest of the room. Like, I'm sure some people did, but it wasn't a normal thing for middle class folks to care much about, as far as trying to make their own house look that way.

There's, relatedly, probably some effect from all those home makeover & house-buying shows, which constantly push "any 'weird' color is bad, and any room without great color coordination is bad, and it's not OK not to mind it". There's a Mitchell and Webb sketch about this, in which two home-buyers on such a show are presented a bathroom with avocado fixtures, and the host berates them over multiple takes because their initial reaction isn't as negative as it ought to be. Fiction, but there's definitely a kind of same-ness to people's reactions to any but a few acceptable colors on those shows.

That seems unlikely. Look at fashion and decor magazines from the past and you see lots of lively colours that are coordinated. If you go house hunting and see a couple houses that haven’t been updated for decades they have a definite palette.

Alternatively, people in the past had fewer possesions (not out of virtuous minimialism as some people portray, but sheer economic reality), and so had more time to consider how each would fit in.

Seems like a stretch that using screens has led to a decline in interior decorating. You could argue a whole generation including men are now devolved into desk setups, plants, terrariums, "earthy colors", hygge, woodworking, aesthetics, etc through Pinterest, Instagram, etc. The same revolution has happened for the glorification of cooks -> chefs and the proliferation of home bakers