Actually, "b" is substantially the same on Mars. Both Earth and Mars have atmospheres composed of "clear" gases--gases which are transparent to most of the visible spectrum--which means that the phenomenon of Rayleigh scattering is the major influence on the color of the sky...absent contaminants like suspended particulates. CO2 is not red, it's clear like oxygen and nitrogen. I argued that Earth's atmosphere should be more prone to particulate contamination because it's thicker and powered by more incoming solar radiation, and I don't see the flaw in that reasoning.

As for "a", intensity of illumination, that doesn't shift color. Colors appear different to us at low light levels because of nonlinearity in the responsiveness of the rods and cones in our eyes--that's a sensor artifact, not any intrinsic consequence of low light levels. Color refers to the frequency of the light waves, independent of their intensity.

In the case of "c", of course--I'd be astonished if the physical laws that produced the rocks of Earth produced significantly different rocks on Mars. If composition of the rock, therefore of dust in the atmosphere, is the same, why isn't Earth's sky red too?

Of course, I'm just trying to puzzle it out using pure logic, and the limit of logic is the lack of knowledge--there could easily be something I don't know. But so far nobody's made a convincing argument that would lead me to expect to see a brilliant dayglo orange sky in every damn official image of Mars. That strains my credulity to the breaking point.

Blue skies, smilin' at me
Nothin' but blue skies do I see
Blue birds singin' a song
Nothin' but blue birds all day long!

Never saw the sun shinin' so bright
Never saw things goin' so right
Noticin' the days hurryin' by
When you're in love, my, my how they fly!

Blue days, all of them gone
Nothin' but blue skies from now on!

< instrumental break >

Nothin' but blue skies from now on!

< instrumental break >

Nothin' but blue birds all day long!

Never saw the sun shinin' so bright
Never saw things goin' so right
Noticin' the days hurryin' by
When you're in love, my, my how they fly!

Blue days, all of them gone
Nothin' but blue skies from now on!

< instrumental break >

Nothin' but blue skies from now on!

~ Words and Music by Irving Berlin, 1927
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