Coloring books are a thing of the past. Kids today aren't as thrilled by a basic coloring book as children a generation ago. That is because these children have been spoiled by all the fantastic new methods of coloring using the computer or with fancy markers and pens. While you might get away with a coloring book and crayons for a long car trip, don't expect your school age children to sit quietly at the table coloring pictures the old fashioned way - there is a something better around, and they know it.
The variety of them that are offered ensures that children of various ages can find something that they would be content with. In fact, you may feel good about composing a list of great sites where various online printable coloring pages can be found. The other option is just to type specifically what you are looking for into a search engine and it will bring up relevant pages to get it.