This montage of images of Saturn's moons (left to right: Prometheus, Pandora, Mimas, Enceladus, Tethys, Dione, Rhea, Iapetus, Hyperion and Titan) was produced by Gordan Ugarkovic from Cassini data in NASA's Planetary Data System (PDS). The moons are shown to scale with each other. The images have all been processed in the same way in order to show the actual differences in color and brightness among them.
Brightness is a function of "phase angle" -- they will look brighter if the Sun is more "overhead" -- so Ugarkovic attempted to choose images lit at phase angles of approximately 50 degrees. (Prometheus and Pandora are much more fully lit at 10 and 19 degrees, so they appear brighter than they should.) Enceladus is clearly the brightest of the moons. Mimas, which is closer to the ring system, is rather darker. Tethys and Rhea are similar in color and brightness, with a slightly reddish or orangish case to their gray surfaces, while Dione is much grayer and is stained dark near the "wispy terrain" at the bottom of the image. Iapetus' Cassini Regio -- its dark-stained leading hemisphere -- is practically invisible. Hyperion is nearly as dark, and even Titan appears dim (though it makes up for dimness by its large size). Unlike the raw images available immediately on the Internet, the data released a year or so later to the PDS includes information on the proper calibration of the images. Calibration allows images taken at different times and of different targets to be compared in this fashion.