Green Switch- In Donut Plains 2 there are some tunnels on the ceiling one of them leads to a place where there is a key hidden.

Red Switch- It is located in the Vanilla Dome. In Vanilla Dome 2 there is a secret area of water that you can get to by using the blue 'p' switch.

Blue Switch- In Forest of Illusion 2 there is a hidden key towards the end of the level.