Saturn is seen in a crescent phase, and the rings are lit from below. The B ring appears relatively dark because it is opaque, while the very tenuous C and F rings appear bright because the tiny particles in them scatter light efficiently to Cassini.
As of July 26, 2006, Cassini's orbit had been shifted so that the spacecraft spent the majority of its time on the night side of Saturn and on the northern side of the ring plane. All of the globe of Saturn that is visible is actually in local night (note the direction of the shadow cast on the rings). It is partially lit by light that is reflected and scattered off the rings, especially in its southern hemisphere (below the ring plane).