Several "obvious" things have been said in this thread that don't actually work in practice.

Yes, there was a callibration target and a coloured flag on the Vikings, for the purpose of imaging things.

IF you assume that lighting conditions on Mars are like Earth (blue sky, hot yellow-white sun) and reference everything to how the flag and chart would look under that lighting, then -surpise, surprise - you get a blue sky.

Real circular argument here, of course. You've calibrated back to your original conditions.

If you take more care about comparing the measured colour intensities on Mars with those obtained with the same camera when callibrated on Earth, you find that lighting on Mars is NOT like on Earth.

From that, you can eventually get a greyish sky with a hint of pink, or ochre, or steely blue, depending on the exact details of what you do.

Meanwhile, the rocks remain bluish grey and rusty ochre, and the dust grey-orange-tan.

I obtained from JSC a packet of their Mars soil simulant which is spectrographically matched to the Viking Lander images. It isn't orange at all, on Earth. Instead, it's a speckly brown-grey-cream.

Don Davis has a nice page on "true" colours on Mars, and every year LPSC has small papers where people have recallibrated pathfinder or Viking images to improve the appearance.

But after a while you get into problems of Mac vs PC colours. Printed vs Screen colours. Sony vs Mitsubishi. And even one specific monitor of the same brand vs another.

The latter problem is a hair-puller for me. We have several twin-head workstations set up with 2 monitors side-by-side. Same brand, same batch, same vintage. Fresh out of the box, the screen colours don't match. When you have two side-by-side the mismatch is clear and cover ranges of green, blue, pink, grey etc. tints.

With a lot of work one can shuffle the monitors around and get better matched pairs, then tune the colours and brightness to further improve, but of course as soon as the students get at them, they fiddle with the settings and it's gone. (And in time, everything drifts, anyway).

In short, we can argue till the cows come home about colour on Mars, and never have a final answer. The only answer is to go there yourself and have a look.

(But for an experiment, try wearing tinted sunglasses or swimming goggles. Even with a strong tint, your eyes/brain adjust to show the "true" colours you expect, so on Mars, the sky might start to look blue if you were there for long enough and never saw Earth reference lighting! [The first sign of insanity in a marooned Mars explorer would be seeing the sky as blue?] )!!!