With online printable coloring pages you never have to keep volumes of coloring books around. Most children change what they would like to color often. They bounce around from one theme or one set of characters to the next. Therefore they may abandon that coloring book with princesses or cars days after you buy it. Printing out the pages though allows them to always get what they want.
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.