I didn't forget about this! It's just nearly impossible to find this information.

Most of the books on non-computer animation are old, and some don't even have indexes. They also have cute, inventive titles for their chapters, making it near impossible to figure out what the chapter's actually about.

There's a lot more info. out there about the early pioneers and the
strange ways that they animate than there is about the actual process before cels were made in color.
JWR wrote:The main reason was when the early animation was in black & white was that that was how the TV's of the time broadcast in. The last daytime B&W tv broadcast was in 1967 and a lot of the early animation was made in the 50's & 60's.
Actually, that's completely incorrect. The "golden age" of animation apparently started in the 30's. Snow White (which used colored cels) first came to the theatres in 1937. So, there was a medium for color animation--the theatre. Animation from the "golden age" was all broadcast in the theatre, most as small shorts after a movie.
Apparently, as early as the 20's, cartoons were being shown in color. However, the film itself was not
filmed in color; it was dyed afterwards. The books I read did not make it clear whether the cels were painted in color or not.
"The film emulsion, being gelatin, could easily be dyed by immersing the film in a vat. Color films were unstable and the colors had a habit of vanishing overnight but a workable color was developed in 1929 and several musicals were filmed using early Technicolor and Cinecolor" (Heraldson 42).
The first color cartoon was done by Winsor McCay; he colored one print of his first cartoon,
Little Nemo, for one of his live vaudeville acts in 1912 (Smith 5). This cartoon was done on rice paper.
The only time I found a mention of color paint and cels, is from "the point in time in cartoon history when color suddenly became inseparable to cartoons . . . . Some cameramen, placing a colored (opaqued) cel on the background and then filming it found they could not remove the cel--it had stuck to the background. Drying and stability were teh main problems. Studios tried many colors, lacquers, and oil and tempera but none served adequately. Also some colors would become brittle and separate from the acetate surface of the cel when dry. The studios thus compounded their own celluloid-adhering colors" (Heraldson 132).
Considering how some of the earliest animation was on stuff like rice paper, there was no color involved. They had to draw everything for each frame, including the backgrounds. Also, looking at how much trouble color gave them when it became a neccesity, I imagine that if they ran into that trouble to begin with, they just decided to skip the color process. After all, they could color the film itself much easier for the theatre productions, and it did not matter, like JWR said, for the TV ones.
Other random, interesting facts I learned:
Some of the earliest animation included filming images on a chalkboard. Pieces of the image would be erased and redrawn to show movement.
Another early animation was done with crude stickmen by a Frenchman who could not draw. Many people in France consider him the creator of animation even though other people in different countries had done stuff before him.
Disney's
100 Dalmations was the first cartoon in the US (and possibly the first ever) to use the Xerox form of transfering cel lines rather than having them hand-inked.
The books/articles:
Heraldson, Donald.
Creators of Life: A History of Animation. Drake Publishers: New York, 1975.
Smith, Conrad. "The Early History of Animation."
The American Animated Cartoon. Eds. Danny Peary and Gerald Peary. E.P. Dutton: New York, 1980. 3-13.