The small magenta spots near the center of the image, labeled H, are coarse-grained iron oxide. The other magenta area at upper left, labeled C, is due to clouds. Yellow and green areas probably contain more sulfate. Ancient lava flows from some of these volcanos also show as red and are relatively sulfate free. The vast areas of blue indicate abundant olivine which weathers so easily that it implies there has been little water and weathering reactions on Mars.
Olivine occurs predominantly in the darker materials in the southern hemisphere.
The above image is a new version from the one released earlier. Both the first release and current release images are smaller versions of the full resolution image which is being submitted to a scientific journal for publication. In reducing the image, many of the small olivine zones were lost, being below the resolution of the press release image. This newer image was reprocessed so the many small olivine spots still show. The olivine spots on this map have been enlarged somewhat for easier viewing.