There are other threads that have mentioned the colour photography issues, but whilse the details are available if you hunt, they do take a fair
I though I should provide a little insight into what is behind the aparent discolouration aspect of the images being returned from Mars by the spirit and opportunity rovers.
Ok, first a description of what the problem is. The general public has been thrilled with the apparent quality of the images returned from Mars, but there has been some puzzlement over the overall red hue in the publically released images.
The following images illustrate this problem;
The Blue Nasa logo as released in spirit images;
Sundial as in Nasa released spirit images;
Sundial as photographed on Earth;
Image seen at JPL press releease with blue sky;
How we see colour
The human eye contains cones that are sensitive to specific frequencies of light that correspond to Red, Green and Blue.
How PanCam sees colour
Pancam is equipped with a monochrome camera. In order to create colour images various filters are placed over the lens. After the images are combined a colour picture is created.
Comparison between the human eye and the pancam
The following table compares light frequency sensitivity of the human eye cones with the filters available to the PanCam on Spirit and Opportunity rovers;
Red: Human eye 570nm PanCam L2 (750nm), L4 (600nm)
Green: Human eye 535nm PanCam L5 (530nm)
Blue: Human eye 445nm PanCam L6 (480nm)
Nasa are producing colour images using pancam filters L2, L5 and L6 resulting in images that are converting blue objects into muddy red objects.
If we use L4, L5 and L6 filters we end up with images that are closer to what would be observed by the human eye if a human observer were located where the pancams are.
Fortunaely, whilst most colour images returned by the spirit rover are using filters colourset that includes the L2 (near IR) filter instead of L4 (Red), I have managed to find one image that uses the L4, L5 and L6 filters.
To my knowledge this is the only image sent back from spirit that truely represents what the human eye would see;
Compare this to how the object looks back on Earth;
As you can see, using this combination of filters produces an image that better represents what would be seen by a human observer on Mars. Taking sets that include the L2 filtered image may well be more scientifically usefull, but it is annoying that this is the only example of it's type that is available to the public that paid for this mission.