Figure/ground illusion
Looking at the "near-true color" image at CICLOPS -- I could swear the dark brownish surface is the natural color, and the white stuff is the deposited layer... especially at higher latitudes, where it looks like it's slumped down off the crater walls to re-expose the brown substrate.
But I've been dealing with deposition of white stuff a lot closer to home, so I'm probably predisposed to misinterpreting the photo.