Coloring books are an activity that was originally designed for children. In recent years adult coloring books offering more mature images and more complicated lines have become a popular item.
Of course there are still some adults who prefer the simplicity of children's coloring books. There are also some adults who prefer turning the children's coloring books into adult coloring books through some truly harrowing means.
It's fine, I suppose. There are worse hobbies to have. Just be sure to never show these to any children.