The Sullivan illustrations are pen-and-ink, all black and white and beautifully done. There is an edge to black and white that disappears in color - the underlying philosophy of the Rubaiyat is pretty grim, and he captures that.
The second illustration has some side jokes - the graffiti on the wall says (in Greek) "Socrates is an idiot" over caricatures of Plato and Aristotle. The Latin legend over the door "Here lives felicity" has been scratched out. It illustrates the verse
Myself when young did eagerly frequent
Doctor and Saint, and heard great Argument
About it and about; but evermore
Came out by the same Door as in I went.