Why do pictures from orbit of the craters at Merdiani show them to have bright white rings, when pictures from the rover dont show this?

This is a bit of an optical illusion. The white ring of material is on the inside of the crater rim. The view of the crater from the ground shows the lighter outcrop (it's actually reddish) on the inside of the far eastern wall. The west rim is lower and the lighter toned material isn't exposed there except for a couple of small patches and in some ejecta boulders.
http://marsrovers.jpl.nasa.gov/galle...-B034R1_br.jpg

This is an almost true color image, but is still too blue I think. Based on the dustiness of the atmosphere at Meridiani, the sky probably should be redder. But that's just my qualitative opinon.