SilverWook said:

I do recall some black and white Warner Bros. cartoons being colorized, (mostly early Porky Pig shorts) well before the 80's. The color versions were less detailed than the originals I finally saw much later, so perhaps they simply traced over the original footage.

Yes, if I do recall correctly, those were sent to Japan and they just retraced and colored some of the animation. That's why they look kinda choppy.

Then, in the mid to late 80s, the process was aided by computer technology, but it's still pretty primitive as you can see if you watch one of the colorized Laurel & Hardy movies.

Now companies like Legend Films use a much more detailed process: they kind of animate the entire movie by hand, using 3d models for people, cars, objects... it looks like a very time consuming process, but the results are beautiful.