Instructions
Build a pool while in building mode like you would normally. Use the pool tool to define the dimensions of your pool.
Press and hold "Ctrl," "Shift" and "C" on your keyboard and wait for a white box to appear in the top left corner of your game screen.
Type "boo|Prop constrainFloorElevation false" into the white box and press "Enter."
Use the "Raise Terrain Tool" to raise the edge of the pool up four levels from the ground level. You wouldn't normally be able to do this, but the cheat code you entered stops items from blocking the terrain tool.
Drag the "Flatten Terrain Tool" from the bottom of the pool to the top to create a rectangle around the pool that's raised up higher than the ground around it.
Zoom in as close as you can to one of the walls of the pool. Choose the wall texture that you wish to use from the "Wallpapers" section in the build mode.
Click on the spot of the pool wall all the way at the top and in the middle of the wall. If the tile doesn't change, then you didn't click in the right spot; move your mouse slightly to one side and click again and keep repeating until the tile changes.
Recolor each section of the pool walls except the corners. Once you recolor one tile, it'll be easier to find the spot you have to click.
Recolor the corners by clicking in the top outer corner of the wall tile. Recolor two sections of the wall--once in each corner--to get the finished look.
Select the "Flatten terrain" tool and drag it over your pool from top to bottom to return the pool its original shape, making it once again look like a swimming pool and retaining the coloration that you just made.
Turn the cheat code off by pressing and holding "Ctrl," "Shift" and "C" and then typing "boo|Prop constrainFloorElevation true" in the white box that appears. Press "Enter."