Here's what we know:
* Early forms existed: Printed images for coloring predate the modern coloring book. Children's magazines and educational materials often included blank illustrations to be colored in.
* Commercial coloring books emerged in the early 1900s: Early examples of these books were produced in the 1900s, with companies like McLoughlin Brothers publishing them in the United States.
* Popularity grew in the 1950s and 1960s: Coloring books became a staple of children's entertainment and education during this era.
So, while there isn't a single "invention" date, the early 1900s is generally considered the period when commercial coloring books for kids first appeared and started gaining popularity.