In regards to the blue color seen in a post-RAT image:

Space Sciences
Blue subsurface on Adirondack rock?
Posted by Robert Clark on 2/18/2004 7:14:58 PM
http://habitablezone.com/space/messages/314637.html

here are some pages that describe the color calibration for the MER images:

Using Color to See More - Looks Can Be Deceiving
http://www.highmars.org/niac/education/mer/mer00e.html

TOP STORY: NASA Is Not Altering Mars Colors.
http://www.atsnn.com/marscolors.html

They explain that the exposure times of each filter image are not the same so the levels should be scaled before combining the images.
How is this done in the image processing program? Is it just a matter of changing the brightness level for each of the filter images?
One possible way to get a more accurate color combination in the post-RAT Adirondack image for instance might be to use the released true color image of Adirondack from NASA taken before the RAT grinding:

January 19, 2004
Spirit Drives to a Rock Called ‘Adirondack’ for Close Inspection
http://marsrovers.jpl.nasa.gov/newsroom/pressreleases/20040119a.html

Notice that compared to this image, the color of Adirondack in the post-RAT image I posted is too yellow.
Look also at the post-RAT image composed by Keith Laney on this page:

http://www.keithlaney.com/spirit_color_images.htm

In this case he gets black for the subsurface of Adirondack but still his exterior surface of the rock is too yellow.
So what I want is too adjust the colors in the post-RAT image so that the color of the exterior of the rock and also the color of the ground matches those in the released NASA image taken prior to the RAT grinding.
In an image processing program such as Photoshop or the freeware program The Gimp, could you open the released NASA image, find the RGB levels of some representative points in the image then adjust the RGB levels in the post-RAT image of points in the same position?
Instead of this, could you use the image processing program to extract the separate Red, Green, Blue images from the released color NASA image, find the levels of R,G,B of representative points and then change these levels of the same points in the raw R,G,B filter images of the post-RAT image?

Bob Clark