Wiktionary
painting by numbers
n. A recreation in which a board containing a picture to be painted is made up of numbered areas, each number corresponding to a colour of paint to be applied.
Usage examples of "painting by numbers".
It was rather like the painting by numbers he'd seen in the toy shops: some areas were numbered for green, some for orange, some for blue, some for red, some for yellow-and, suddenly, there it was!