One thing I'm surprised the article hasn't mentioned is that coloring is also a profession, and there are people whose careers center around coloring line art (

One thing I'm surprised the article hasn't mentioned is that coloring is also a profession, and there are people whose careers center around coloring line art (typically using Photoshop and a Wacom tablet). They're called colorists (and some people are now saying "color artist"), and they're core parts of a typical comic book creative team. They get their names listed on the cover and everything. Some of these new coloring books for adults may actually serve a very practical purpose: to provide practice material for potential colorists. Speaking of comic books, some of these new coloring books for adults are put out by Marvel Comics. What makes these books "for adults" is that they are filled with un-colored line art from actual comics Marvel has published, and they cherry-pick pages with high levels of detail. You color in one of those books, you're replicating work that's been done by actual professionals. This isn't kids' stuff at all. It honestly makes me wonder if Marvel is publishing these books specifically as a recruiting tool for colorists.