"The idea of offering beautiful and sophisticated illustrations for grown-ups to color simply made sense to me," Marotta told BuzzFeed. "As children most of us will have enjoyed coloring as an activity and a way to be creative, why should we stop simply because we have grown a little older?"
Now there's a sequel to her first coloring book, Tropical World: A Coloring Book Adventure.
Marotta said readers and artists are into the coloring books because it's an accessible way for them to be creative and de-stress.
"Most of us naturally enjoy creative activities, especially those which provide us with some kind of outcome at the end, we like that sense of achievement and the satisfaction that we get from seeing the results of our efforts," she explained.
"Coloring intricate images encourages us to focus the mind and concentrate on the here and now, offering a much needed distraction from all those things in the grown-up world which may be causing us worry, tension or stress, a little bit of escapism."
The author created designs like these for adults to color because she thinks it's a great opportunity for people who want to be artistic but don't know where to get started.
"A blank page can be quite a daunting thing so having an image there ready to start working on can be very encouraging for those who otherwise might not have any kind of creative outlet in their lives," Marotta said.
"I hope that readers, whatever their reason for coloring, will feel inspired by the book and will continue to explore their creative side."