Talking DC and Marvel: In my day, comic books colors were flat, and limited. White people conveniently were approximated by red dots on newsprint. No shading. Now, it’s all airbrushed and very intricate, and some artists use very quirky page lay outs. Did the change come instantly, or was it gradual amongst the titles? Was it simultaneous with DC and Marvel? Were any fans aghast?
I would hazard a guess and say that changes came about when a new technology became common, affordable…and didn’t interrupt the flow of the story.
Pretty safe guess there, Czar 
This doesn’t directly answer your questions, but this is an excellent primer on the history of comic coloring.
I actually taught this at the college level.
It was a “Pre-Press for Graphic Designers” class, and we went over many different printing processes. I passed out comics from different eras and the “kids” enjoyed making fun of my beloved-yet-poorly-colored comic collection.
Nutshell: Early technology (look up “rotogravure”) meant a colorist just picked from a few %'s in four colors (Cyan, Magenta, Yellow and BlacK). Which meant less than twenty different colors.
(So, yeah, caucasians all had the same 10% Magenta, 25% Yellow skin.)
All printed on cheap, low-cotton, high-wood fiber paper (so it was all “pulp fiction”). Which has since yellowed quite a bit…
Once better paper and printing (“Offset Lithography” for the win!) came along, you could get hundreds of colors and gradients.
Add computer graphics into the equation, and you get modern comics with more realistic coloring, subtle shading, and cool effects.
Google “History of Comic Printing” or “Comic Book Coloring” for lots more.
eta: Or read Darren’s link (which appeared since I started typing…)!
Half the equation is the substrate. When the price of comics boomed in the 1990s and moved to specialty stores and sold as collectibles, newsprint was replaced with the coated paper once reserved for covers. With coated paper that was slicker and didn’t yellow, you could expand the color palette from around 50 to infinity. Both DC and Marvel had already produced some high priced prestige titles in the 1980s. As soon as comic book store customers demonstrated that they would pay higher prices for new comics, the technology was already there to make the transition.
I remember both companies making a big deal about “Baxter paper” in the late 80s, and printing some of their more “important” books in that format, with corresponding higher prices. I don’t know exactly what Baxter paper was, but it definitely seemed thicker and rather more glossy than the standard newsprint.
Don’t know if they still use that kind of paper, or if they’ve moved on to something even better. But that was the first “improvement” that I remember that they made a fuss about.
Being a comic collector/nerd back in the day, it’s my opinion that Image Comics helped pioneer the push to greater colors in comics.
Image Comics was built on the principle that creators own their work, making all licensing and creative decisions themselves. This contrasted with the model at companies like Marvel and DC, where the publisher typically owns the characters and intellectual property. Since they had more creative control, they instituted greater quality.
They played a significant role in improving and standardizing comic book color in the 1990s. They also helped popularize the use of fully digital coloring and more advanced color separation techniques, which pushed the industry forward.
They make it in the Baxter building.
I have a college friend who was (and still is) and penciler for Marvel and DC. He started getting professional work back in the early 80s and we would hang around his apartment suggesting cute things to put in the background.
He commented that different colorists worked very differently, and he’d have to adapt his work to mesh with theirs, especially the non-traditional page layouts. What works for one artist won’t work for another, it was a real team creative process.
They make it in the Baxter building.
That Willie Lumpkin ran a hell of a paper mill …
But seriously, folks, some of the “premium” papers (Baxter, Mando) weren’t coated, they were “highly calendered”. *
Short version: to get the ink to sit on top of the paper instead of soaking in, you don’t have to go full “coated”… which I think is too glossy and makes for colors that are too gaudy for my tastes.
*The final rollers as the paper’s being made squeeze the paper, and can be high pressure with really smooth rollers.
That gives a smooth surface, with nice colors and detail.
And if you want a “laid” or “linen” finish, as in stationery, you can use textured rollers.