NASA has indeed altered the color in this picture, a charge which they do not deny. In fact, they have freely admitted it. The original picture was black and white. Neither Spirit nor Opportunity have the capability to take true color photographs.
Instead, what happens is the landers take grayscale photographs filtered to various colors - one image for red, one for blue, etc. These images are then recolored and compiled to create an approximation of a true-color image. The reality of the color may be quite different. In this particular image, the blue tab appears pink because it reflects infrared light, which we cannot see but the Mars landers can. Thus, the colors in the image are at best an approximation of what one would see if they were really standing on Mars.
For the record, the blue tab in the original picture is not a true color image, either. It is also only an approximation of the tab's true color.
This is what I had assumed.
I was thinking, Why would someone smart enough to edit the pictures to remove the green tab leave the other colors different? The answer... They wouldn't. Why would NASA go to the trouble of faking these things if they aren't going to do it right. If I was faking a NASA mission, I'd do it in a sealed class 100,000 cleanroom environment with lots of computer simulations run first to get everything right. I wouldn't allow people to just wander about the set or mess with the lighting. You guys make it sound like the whole thing was staged by a high school drama club, not a multi-million dollar agency full of PhDs.