What is "color by number"

alt. 1 (context usually used attributively English) 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. 2 (context uncountable English) A technique that applies such exercises, as used until recently in the production of large-scale animations for instance. n. 1 (context usually used attributively English) 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. 2 (context uncountable English) A technique that applies such exercises, as used until recently in the production of large-scale animations for instance. vb. To complete such an exercise; to carry out such a technique by filling in each space with the appropriate color.