[Monty22]

Interesting stuff. I just hope we get more probes there, without failure.. And humans to tell us the color.

We need that.

[Hunble]

"We don't need no stinking humans!"

Give a scientist a low cost optical spectrometer on a Mars robot and this question would be solved in less than 30 minutes.

Mie scattering is the key to this subject. Be advised, this is an advanced topic and there will not be much available on the internet.