Construction Foam Wall
These walls are the product of Construction Foam.
When the CF turns into a Wall, it'll be grey.
But with a Painter, you can easily change the color of the CF Walls.
Construction Foam Walls cannot be crafted-- they can only be created by placing Construction Foam and waiting for it to solidify. They also cannot be collected by breaking them with pickaxes; even the formidable Diamond Drill causes these walls to collapse into nothing when broken.