Okay, I give up... I would be grateful if someone could explain to me why the answer is whats it is.
Viewing post in Feline Crime Files: Operation Vulta comments
Not really good with written explanations, but here goes:
The code is based on the 5 ingredient recipe from the grandma story.
In the illustration, the amount of an object is a number.
The color of each object tells you the order to place the numbers in.
You find the correct color order by looking at the color wheel poster, which is read like an analog clock. (By hue you get: yellow [2 moss], cyan [6 droplets], blue [9 blueberries], pink [4 raspberries])
The last digit is 1, because the story ends with the grandma herself.