Magic colouring books - Happy Birthday! (0 colouring page)

This is a hidden drawing, the pattern appears "magically" as the child colours. It can also be referred to as paint by numbers.
The colouring has a code: a colour can correspond to a number, a letter, a word, an operation (addition, subtraction, multiplication, division), or even a shape.
Magic colouring is both a fun and educational activity.
Here is a selection of number, letter, or operation colourings to personalise and print: