The drawing is a two-tier cake with scalloped frosting on a plate, three lit candles on top, divided into 43 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.