I come here to praise the humble coloring book. This type of book preceded my birth, continued after my childhood, and still holds market share today. These books have created billions of hours of free time for harried adults, and countless hours of occupation for their children.
Most coloring books are 8.5" x 11" in size, and about 48 pages long. (I made that number up.) All contain basic line drawings with no color, printed on newsprint, making them exceedingly cheap to produce. The only colorful thing about them were their bright colors. Some books were thicker, being labeled jumbo coloring books, and some were legal size, or even larger, also counting as jumbo. (The coloring world is not known for its killer categorization.) Some are small, often being sold with some sort of crayon or pencil as a complete set.
Many coloring books are generic, featuring objects or people. Sometimes they contain special features, such as mazes, puzzles, matching activities, or even stickers. Most coloring books had some sort of theme or character, so you might get a Christmas coloring book, a Muppet Show coloring book, or even a Muppet Show Christmas coloring book. The licensed characters would usually be popular cartoon characters, TV shows characters, or comic book characters.
The preferred medium for coloring was the humble crayon, almost always by Crayola. The most common type of crayon set contained the basic colors, about 10 crayons, although bigger sets were available. Some contained 24, 48, 96, or even 128 different shades. The bigger boxes came with their own crayon sharpeners built into the base. However, the most popular form of the crayon as the assorted bag/box/container of broken crayons. Every school and house had these, because kids are pure murder on anything breakable, and crayons exactly fit that definition. One of my teachers would melt down the remaining crayon bits into multi-colored coloring bars. To use second graders, that was just too cool.
Pencils were sometimes used on coloring books. The main issue with pencils is that they are more expensive than crayons, lighter marking than crayons, need pencil sharpeners, and were not as comically over-available. They also weren't any cooler.
If you wanted cooler, you got markers. As markers wore out quickly, mostly because you left them uncapped to dry out, but sometimes because you actually colored them to death, markers had a far higher status that crayons. Their colors were bold, but at the expense of bleeding through the newsprint, sometimes bleeding onto the next page, and far too often, drying out as you colored with them. They are also expensive enough that they don't get replaced as quickly. And if they did get replaced, you wound up with a zillion of your least favorite colors because you never used them, and your favorite colors just kept wearing out.
If you wanted to ruin your coloring book, you used watercolors. No child knows how to use watercolors. They smash their brushes into the watercolor blocks and then try their best to paint on newsprint. This is a recipe for failure. Watercolor always loses against newsprint unless its particularly heavy.
All kids start out coloring by scribbling. I'm sure that psychologists describe this in detail, but I'm no psychologist. Eventually you get it through your head to not scribble, which took me extra long. I was a scribbler for far longer than is proper.
Most kids get to the "reasonable colorer" stage, where you color in the lines and generally keep yourself busy. Inevitably, there were those kids who went ABOVE AND BEYOND in their coloring, making you look bad. They would add shading, texture, extra details, and other show-off sorts of things. Those kids were EVIL and BAD and HORRIBLE, mostly because they made you feel inadequate and not up to snuff.
At some point, you stopped coloring. There's no real reason why, but you do. My guess is that the pictures just aren't interesting. There a woman recently who produced some adult coloring books by creating absurdly complicated pictures, and what do you know, they sold like gangbusters, giving the hardcore coloring aficionados something to be an aficionado about.
I also have to suppose that at some point, the seriously artsy kids got into painting, drawing, or something like that. They continued their enjoyment of the visual medium that is coloring, while not-so-interested kids drifted away.
The people who get to come back to coloring are parents, because where there are kids, there is coloring, and sometimes, you suggest coloring because that's what you want to do.