Instructions
Pick up the Floro Spout after fighting O' Cabbage. Go through the pipe and leave through the door. Head back to the Save Block, and jump to the pipe above it. Through the pipe, climb out of the pit and move to the right. Equip the Floro Spout by bringing up the "Important Things" menu. Go through the door, past the scanner, and then through the final door into the gallery.
Move to the right and look up at the first painting. You should note the color of these portraits. Change to 3D mode and you will notice that there is a crack in the floor underneath the painting. Put Boomer on the crack, and then you will fall through into a pit. Switch back to 2D mode, and jump up at the "!" block above you. When you hit it, it will change color. It needs to be black because that is the color of the portrait above it.
Go back upstairs and walk over to the next portrait. You will notice that this painting is white in color. Repeat the process- switch to 3D, blow a hole in the floor and change the "!" block to white. You can also change colors of blocks using Thoreau.
Repeat the process with the final two portraits and blocks. The third portrait is blue in color, and the final one is red. Each one is the same process; Boomer can blow a hole in the floor, even though there is no crack underneath the fourth portrait, and the "!" block has to be changed to the relevant color.
Complete the fourth block and wait for the door to appear. Through the door, the game continues, and you have to face the fearsome King Croacus.