Mars with a blue sky is a bit disconcerting (here’s a picture of a piece of Mount Sharp with the colors balanced correctly). But this is more than just a (ahem) curiosity. Geologists train under the lighting conditions we have here on Earth, but the Sun is fainter on Mars, and the sky a different, butterscotch color. That can trick the brain, making it harder to spot subtle details or see features that would be obvious here on Earth. So sometimes scientists fiddle with the pictures a bit to make them easier to analyze.
I’ve done this many times myself with Hubble images. There might be faint material surrounding a bright star, and it’s hard to see because the star is blasting away. Astronomers commonly change the contrast from a linear scale—where something that’s twice as bright is shown that way—to a logarithmic scale, which goes by factors of ten. So an object 10 times as bright as another “in real life” is scaled to only look twice as bright. A factor of 100 is displayed as a factor of three. It’s actually more complicated than this, dealing with greyscales and such, but that’s the idea.
And that’s how your eyes work, more or less. If something twice as bright looked twice as bright to you, you’d hardly see any contrast at all. Our other senses are that way, actually. We use decibels for sounds, for example, and that’s a log unit too.
Anyway, fiddling with images is a tried and true method to help scientists understand what we’re seeing. You have to be careful and not see something that isn’t there, but getting trained to do that is easier than trying to see something you just can’t see.