In Donut Secret 1, swim up into the first pipe to find a room with coins and a P-Balloon. Notice that the pipes in the room are orange. Collect the P-Balloon, and float as far up as you can. When you come back down, the pipes will have changed to green!

Depending on how high up you float, you can change one-third or two-thirds of the pipe to green. Using the right wall as a measurement guide, stop at about two or three blocks below the group of two coins near the wall. When you come back down, only the bottoms of the pipes should have changed to green.