NASA has been caught modifying the colors of the Martian Sky from the Pathfinder mission.
Forum user Ski posts the following on the topic of Mars:
"I had no doubts after the infamous sun dial pictures. It is clear that the "Mars" sky is "sky blue". No doubt somewhere remote in the Atacama desert not too distant from the observatories there."
There are several tell-tale signs that colors in the photographs have been modified. For example, in the Pathfinder record we can see that the NASA logo has turned a bright pink.
Also, in the 70's there were allegations that NASA was faking the color of the Martian Sky from the Viking missions.