I decided to post a walkthrough for the game because maybe some puzzles are not as obvious as others and specially if you play the game with kids, that can be frustrating.
So here it goes:
First you need to find all 5 colours. Yellow is in the treasure chest in his Bedroom. For the code look at the notice on the chest. The Numbers are the Numbers of books in that colour that you can find in the shelf (232).
Then proceed to the kitchen, disguised as ketchup andmayo you can find the white and red in the fridge. Also take the sausage out of the fridge. Give the sausage to the dog. He leaves the bed and you can take the black.
Take the stepstool from the kitchen. Take the scissors from the kitchen counter.
Go to the bathroom and use the stepstool with the cupboard. You can take the blue and have all 5 colours.
Next use the stepstool with the painting in the floor to take it. Use the white on the painting to get a blank canvas. Put it on the easel.
Take the dragonposter of the wall and use it with the scrissors to cut it out. Put it on the canvas.
Go to the sisters room and ask her for help.
Take the towel from the bathroom and give it to your sister to imitade mona lisas robes.
Make your sister smile by using one of the paints with your face.
Voilá, you have done it. The parents will be impressed ;-)