If you want to change the color a wall (or all the walls) in your project, open the 3D View window, and focus on the wall you want to change (or any wall if you want to change them all) .
Click the wall (or tap your finger if you're using a touch device) to select it. When a wall is selected, it will be outlined in green (just like the case for selected 3D objects) as you can see in the following image:
Once the wall is selected, go to the Properties window and select Material type (highlighted in red in the previous image), opening the list of available colors for the walls:
Click any color, and immediately the wall will change to that color. Click Apply to all if you want to assign the same color to all the walls in the room.
In the following image, yellow has been assigned to all walls in the room:
