Here's what I see: a composite picture made with science cameras, that is, cameras that are using filters to see in specific wavelength, extending beyond that of the human eye. Using this data to create an approximately "true color" image is actually rather difficult, and depending on which filters were used, it may not even be possible to get very close to what an eye would see.
The real problem I think is that nasa does not spend much time educating people about the process involved in making (not taking) these color pictures. What people want is what their personal camera does, they want to see on the picture what their own eye would see if it was there, but it all comes down to a choice of priority: do you drop one (possibly several) of the science filters so that you can make prettier pictures, or do you keep the science filters and tweak your way into making (necessarilly) approximate color corrections ?
I think we should keep all the scientific filters we can fit in these "science machines". What I would like to see is another mission, dedicated to the human aspects of mars. ie, taking pictures in the human visible range, recording the sounds of mars, perhaps even reviving the idea of the mars glider, which has been scrapped a year ago or so. I'm sure there is a ton of fascinating and beautiful things we can do on mars that have no scientific interest whatsoever.
It would be a hard one to fund, of course, but I think that'd be the way to go. In the meantime all we can do is have crude approximations of the colors of mars. That's not to say that some approximations aren't better than others, and certainly, some of the picture composite that have been made by others than nasa/jpl tend to be better corrected (as the calibration target confirms).
If there was a big conspiracy about it tho, I wouldn't expect jpl to make the raw pictures available at all... everybody who's been making their own "true color" versions of the mars panoramas has been using these.
I'll grant you this, It does looks like there isn't much motivation at nasa/jpl to get color perfect composites. Perhaps that is because they know that's not possible anyway with the dataset they have, so maybe they just get it to look familiar enough... clearly they could try harder... it _is_ important to the layman, and by not doing so, they open the door for Hoaxland & al to cry conspiracy.
ps: Congrats on your new book, btw :-)