There are lots of tools: pencils, markers, paint crayons. Each material has its own characteristics and advantages, promoting the development of specific skills. It is recommended to start with pencils.
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.