Most of the rock surfaces in Gale Crater are covered by dust, making many photos have very low contrast and with little variation in color. By stretching each color to near the full range available, small differences are emphasized. The bluish tones here are not near the true colors, but are blue only in comparison to other surfaces; because the dust is redish, blues and greens are stretched more.
Most of the variation appears to be due to differing thickness of dust on the surfaces.
These Curiosity photos are from the mast cameras.
Original image, taken: 2012 SEP 29 09:12:44 EDT