My first encounter with meditation was when I became ironically Buddhist in high school. I grew out of the faux spiritualism but did not stop meditating. There were too many benefits. It felt good. If doing math problems or writing is exercise for your brain, meditation is stretching. There is a wealth of scientific evidence in favour of meditation. I definitely notice that concentrating and focusing in work or school takes on a meditative quality.
How do you meditate? Simple. Sit down in a comfy spot somewhere quiet. Close your eyes, but change nothing else. You don't need posture to meditate, you don't need special breathing. Just be comfy. Count your breaths from one to ten. If you think about something else (my toes are cold) just restart your count. You might find it hard to get to ten. That doesn't matter. Don't focus on the goal.
What you are doing is honing your concentration. Do this for ten minutes, or thirty, or until your kettle goes. Doesn't matter. Every time your mind goes somewhere else you notice, and gently nudge it back. Eventually this becomes second nature. There is another type of meditation where you use this focus to direct your mind to something, a problem, a sound, a thought.
So how does this relate to coloring books? I bought a coloring book and a pack of colored pencils a few days ago and while I leaned over my kitchen counter halfway into a page I felt a familiar feeling. It was the feeling of calm, sure, concentration. The feeling of meditation. I feel this all the time. i do it subconsciously at work. I do it when I bike or walk.
I've always said that more people should meditate, that if it was a part of our culture it would improve everyone's life. Adult coloring books are the same exercise. You don't count your breath, not that kind of meditation. The other kind. You focus on what colors to use, how to use them. All while staying in the lines.