The good news is that since its remarkable come-back there have been several apps that one can download and color on their mobile or tablet devices.
I have tried two, both Colorfy and Color Therapy and have voted to recommend that later. Color Therapy offers more of a challenging coloring page with various color books to choose from. Not only is it more practical, it is cheaper. You do not have to go out and buy a book and crayons or markers.
There is however a catch. After you see their options you get roped into wanting to use all the color palettes, in which most of them are locked. You can like their social media and open up a few of them, however you are going to end up buying the major package that supplies it all.
I eventually caved in and purchased the in app package because it has a nice gradient option that allows your coloring photos to come to life. I highly recommend purchasing the extra package because it really does make a difference. The total in app purchase came to $7.99.
I have to honestly say I do get very involved in my coloring pages and find myself loosing track of time. Therefore my last piece of advice is to make sure you give yourself at least an hour to color before moving onto the next task of the day. Your coloring page can always be saved and you can return to it.