Old ­school coloring books are hot items in bookstores and even supermarkets. Sales of grown-­up coloring books are estimated to grow again this year after 12­ million were sold in 2015.

Bill Seaver's Smith­ Seaver Publishing released its first coloring book for adults in November and it was so successful the company will release 20 more this year.

"One of the reasons I think adults are flocking to coloring books is there is a calming effect," Seaver told us. "People are so busy today and they're so connected and wired to everything they do in their life this is a way to sort of disconnect and do something that's analog for the first time in a while."

Business has been good for book publishers again. Sales of colored pencils jumped 26% in 2015.

There are also digital options. In iTunes and the Google Play Store, dozens of adult coloring book apps are available. Many of them are free but offer more color choices as in ­app purchases.

Most coloring apps are a simple fill­-in­-the-­blank with color just by tapping on the screen. A small number actually respond to a stylus or finger the way one would actually color on paper.

In our test of coloring book apps none come close to the experience of using old school crayons on paper coloring books.