The drawing is a curling woven horn with round fruit spilling from its mouth, divided into 40 enclosed areas. Each numbered area takes one of six colours, listed in the key along the foot of the sheet; areas too small to carry a numeral are left blank and stay white.
The numbers are placed by finding the areas themselves rather than by labelling each drawn shape. Where two outlines cross they make a new area, and it is that area that gets the number, so there is never more than one number to follow in a single patch.
Six colours, not twenty: the sheet is meant to be finished with the pencils in an ordinary tin.
Every boundary is a closed contour, so a fill tool stays inside the area it was clicked in and a coloured pencil has an edge to stop at.