Nasa has been accused of digitally "tweaking" drab brown scenery to make it redder. It has even been suggested that Nasa removed green patches to hide evidence of life.
The theories gained credence after Nasa told New Scientist that "getting the colours right is a surprisingly difficult and subjective job", the magazine reports today.
Most of the pictures have been taken through green, blue and infra-red filters - instead of green, blue and standard red filters, which would have produced more accurate colours.
Dr Jim Bell, who worked with Nasa on the Mars rovers' cameras, said infrared filters were used because they helped geologists to distinguish rock types.
In reality, Mars appears red largely because of the dust in its atmosphere.