The material. Some coloring books offer the ability for the child to simply run over the surface with the pens or crayons provided, and it will turn out to be the right color regardless. Some are more like paint books, and some have magic marker surfaces and come with markers that will not show up on any surface but the specially prepared book pages. Choose the book that best suits you and your child. If you are worried about markings on the walls, then dont select a pack of basic crayons, get something that uses special crayons. If you want your child to exercise their own creativity and not have to fit in the lines or color schemes planned, get basic pages. It is up to you.
Those that really like to color are going to be looking for printable coloring pages all over the place so that they dont have to drive all over creation looking for that special coloring book. If you and/or your kids really like SpongeBob Squarepants, you will like looking for SpongeBob coloring pages so that you can make your own custom-made coloring book or books from SpongeBob printable coloring pages. Are you aware that there are a good many adults that really love to sit down and color when they have a few moments to simply sit down and relax? This is something that you might not be aware of, but really need to think about because it happens every day somewhere in this big world of ours.