All color pictures show here taken by Spirit are false-color. Mosaics or panoramas released by NASA, which can be found in their archives, show the surface and sky in a range of colors including an overly saturated shade of reddish/rust. In some images like the one above the color temperature was adjusted while working towards the goal of improving detail but in the process it made the sky appear in a lovely shade of blue.
For an example of some of the variation in the colors of Mars images directly from NASA, take a look at the small screenshot excerpts below. Note, popups are no larger than the thumbnails.
https://kilroyonmars.com/75-analysis/298/a-note-about-color#sigProIdd159f1262c
The seemingly simple task of creating a color image is more complicated than you might think. Spirit is only capable of creating color images using its black & white cameras with color filters and this requires processing back here on Earth. To see these raw images click here. Since the filter sets were selected to provide specific scientific information rather than being optimized for color photography personal judgement and interpretation is required to create a an approximation of true color. You can see the application of artistic license in some initial panoramic images which were rendered in an overly saturated red. In contrast, images like the one at the top of this article have an altered white balance and as a result yields a lovely Earth-like blue sky in the process — which is not correct either.
The current Curiosity mission is equipped with greater number of more advanced cameras including one that can capture high resolution images and video(!) at 10 frames per second and are better able to more acurately render colors. See JPL for perspective and information about the cameras.
An analyst born 53 days before NASA. A midwesterner now living in the southeast. 
