[bradleyland]

The NASA page has much better copy about the object:

    Much of the rock is bright-toned, nearly white. A 
    portion is deep red in color. Pinnacle Island may 
    have been flipped upside down when a wheel dislodged 
    it, providing an unusual circumstance for examining 
    the underside of a Martian rock.

http://www.nasa.gov/jpl/mer/rock-pinnacle-island-pia17761/in...

From this, I get the impression that the other rock you see in the photo is actually bright white as well, but is covered by red martian dust. This is probably the most interesting thing about the whole photograph.