Were I doing one here where I live I’d use a lot of perennial flowers and herbs.  Right now my sage plants are in full bloom and they make it through our dry summers with almost no water.

Columbines on  a north slope might be nice.  Very colorful right now and bright green the rest of the time.

You could plant a variety of thyme, have a nice green roof most of the year except when it bloomed a beautiful lavender.  Or you could plant a thyme with yellow leaves.