I have figured out how to embed artwork in the earthcrete or concrete walls of the greenhouse.
First you make a sketch of the artwork on your inner form board. Then apply colored concrete to make the sketch into a 1/2 inch thick layer of concrete. I have figured out how to use a calk gun to do this. Also add other inner fill colors if necessary after the initial layer is dry.
Next you add a layer of concrete mix over the completed artwork and fasten on a couple of boards as the artwork can slide off the form board when you lift it up. As you fill in the concrete wall, you remove these boards.
When the concrete or earthcrete wall is dry, remove the form boards and your artwork is now visible. If you look closely you will see the lines from the layer of concrete that held the artwork as the forms were filled.