Look at the piano. You'll notice that there are white notes and black notes. Figure out the difference between them and you'll be able to make whatever kind of music you want.
Gershwin's melodic gift was phenomenal. His songs contain the essence of New York in the 1920s and have deservedly become classics of their kind, part of the 20th-century folk-song tradition in the sense that they are popular music which has been spread by oral tradition (for many must have sung a Gershwin song without having any idea who wrote it).
Porgy is...an interesting example of what can be done by talent in spite of a bad setup. With a libretto that should never have been accepted on a subject that should never have been chosen, a man who should never have attempted it has written a work that has a considerable power.