Later today I'll be posting the results of contacting Justin Maki of the MER imaging team, reading the copy of his 1999 paper that he kindly sent, and other research into the question of "the true color of Mars". Turns out that it is possible to do a reasonable calibration of the images, based on the published ground calibration factors, using custom software that essentially inverts the autoexposure process that took place on the spacecraft. Based on that, I've come up with a method to, first, process images Dr. Maki's way, which I call "TrueColor(natural)"; and then to "loosen-up" to create images using "TrueColor(augmented)", which retains a valid color balance but allows a wider range of colors.

In the meantime, here's that image processed in TrueColor(augmented):