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.
It is believed that the company The McLoughlin Brothers (en.wikipedia.org/wiki/McLoughlin_Brothers) invented the coloring books. They published a book called The Little Folks' Painting Book in collaboration with Kate Greenaway in 1880.