Just like guided meditation, coloring allows us to switch off our brains from other thoughts and worries and focus only on the moment, helping to alleviate stress and anxiety. It can be particularly effective for people who aren't comfortable with more creatively expressive forms of art, like painting or writing. The participants who are more guarded find a lot of tranquility in coloring images. It feels safer for them and it creates containment around the coloring process.
When I was a kid I just loved Superheroes and cartoons. It's no surprise then that my favourite thing to do was to watch Spiderman Cartoons! I would plan my whole day around them to make sure I didn't miss them, often running home from school just to see them, even if I knew they were repeats.
As a parent I know how expensive purchasing hundreds of coloring books can get and its such a shame to let something so fun pass them by because of money, so finding free pages online are a fantastic way to still provide kids with the fun without the expense. Let your kids evolve, grow, imagine, and participate in one of the easiest methods around to challenge their skills with coloring.