color by number

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun usually used attributively A simple drawing exercise, intended especially for children, consisting of a line drawing that divides the artwork into distinct spaces, the intended color of each indicated by a number and referenced from a key.
• noun uncountable A technique that applies such exercises, as used until recently in the production of large-scale animations for instance.
• verb To complete such an exercise; to carry out such a technique by filling in each space with the appropriate color.