Wild goose chase
That image was created from right-eye images, which means it had to be composed of one ultraviolet image and one (or more) infrared. I wouldn't expect much in the way of "true color" from an image like that. I'm surprised NASA would even use the term.
I've looked at the raw images behind Daniel's effort, and there's no question that there's a lot of greenish-blue there. But that's already pretty much settled for the region Spirit's travelling through.