when you use the round glass (soap + dirty rock) on the inscription on witch's house, you read: 1 entrance, 2 blue light, 3 yellow friend.
What I've figured out: these are hits to solve the witch's attic puzzle.
1: number behind picture in the entrance. 3: number tatooed on the butt of yellow fish. 2:

?
I found the words "blue=ant", so I'm assuming I have to shrink to take a good look at ants to find this last number?? :

Already have shrinking potion.