It is easy to find printable color-by-number worksheets online. As of 2015, an online search reveals multiple websites offering free printable color-by-number pages, including All Kids Network, Education.com, Crayola, RaisingOurKids.com and Preschool-Printable-Activities.com.
Preschool Printable Activities says that, in addition to being an activity that children generally enjoy, coloring promotes hand-eye coordination, and coloring by number teaches children to follow directions.
The color-by-number worksheets available on the Education.com website include pages that require students to perform basic mathematical operations, and the answers to the problems indicate the colors for the children to then use. The worksheets have themes such as butterflies, rainbows, vowel sounds and rockets.