Tiling the courtyard challenge in Scheme / racket

The challenge was to most efficiently tile a 2d plane using L shaped pieces like these Where the plane also had a single grid tile that was already filled, anywhere on the plane.
The approach of course was to use recursion, but in a fashion that was smart and scalable.
I really tried to make the code as readable and simple as possible, a straightforward execution of a planned and thought-out recursion algorithm. Overall I'm pretty proud I did it so quickly, especially in Scheme, which might be the most interesting language I know.
Also quite proud of the rendering, very nice colors I think.