re: Same old objection...
Another factor in getting accurate Mars color is the changing amount of dust in the atmosphere. When Spirit landed the opacity (tau) for Gusev was around 0.6, now it is down to about 0.2, IIRC. So there is still a reddish filter being applied, just not as much as before. Also, I think that tau at Meridiani at landing was ~1.1 and has now cleared to about 0.5-0.6. Presumably additional clearing will occur as water ice clouds are increasingly forming.
For calibration, I recall JPL saying that tau=0.1 is a clear day in LA, and 1.0 would be very smoggy.