Thanks to colorings the child perceives the world pattern, implements its own color discoveries. If a child paints the picture in unusual colors, you should not abuse him for that; it is his own opinion.
There is another advantage of colorings for adults. Most of them are very nice and include impressive black-and-white contours that can be turned into a work of art. Imagine the feeling of satisfaction that you get when you have finished drawing and will be able to appreciate the beauty of the masterpiece created by you personally.