The tower tileset is not normally selectable in custom levels, but you can customize the graphics assets for your level to make other tilesets look like the tower.
Which level did you see the rainbow background in?
I saw it in a Level called Cloning Technology, I was really confused when I first saw the rainbow.
One little note by me: I chose not to have the actual tower tileset, I just want to see how it's done.
Edit: I will put you in the credits too!
what the hell why am i talking about credits lol