Just three points
1) Some of the more recent images released by the JPL show a blue sky
2) The Hubble photos show, along with plenty of cloud, white haze over large areas; shouldnt that lead a whitish-hazy sky? The opaquity of the atmosphere should be related to water-ice or CO2 ice haze, not just dust. The Martian atmosphere may be thin but it is never far from saturation.
3) The Hubble photos, and also amateur photos of Mars, show a blue colouring near the limb – wouldnt that also indicate a blue sky?

I really wonder – I’d be surprised if the Martian sky was the same bright blue as the Earth’s (although hazy skies could lead to that effect) but I am not comfortable with the official photos either, as if the surface is under a permanent dust storm (if there was that much smog, how can we see the surface?). The excess pink/ bright orange cant be right (the actual space probes are totally mis-coloured in official photos!), so I am left in doubt.
I dont mean to sound daft but why cant they send a cheap digital camera that would take normal photos without needing to “calibrate” the images from multi-million dollar cameras?

Does anyone know what the real colour of the sky is?

The spectral composition of the incident sunlight of mars is known. This spectrum is also smooth.
If the background of a spectrum is smooth then even sharp peaks don’t have much impact if they don’t contain very much energy compared to the rest of the spectrum.

The reconstruction of the colors of the colors of photographs is possible because we know the spectral composition of the Martian sky (the color that this article is dealing with). If you know the illumination by the sky you can use this knowledge to “fill the gaps” of sparse filter sets by reverse engineering because the materials (which are known) have smooth spectra and are lit by a known light source (direct illumination by the sun, indirect illumination by the atmosphere).

Nobody said precise reconstruction is always possible!

But as long as you know the material or the illuminant you can reconstruct a good approximation.
In the case there are enough data and facts known to find the appropriate colors! Finding the colors does not necessarily involve a full spectral calculation. The band-pass filter peaks only need to be at the right places (in case of the Mars photos these are the filters L4, L5, L6). Then the whole works similar to what the human vision system does. Human vision collects photons only with three kinds of color receptors, each of them acts as some kind of wide band-pass filter, collecting photons over a wide range of wavelengths. Color perception is a 3-dimensional phenomenon, so during the “calculation” of color the infinite-dimensionality of spectra is reduced to only three dimensions – If you choose Lab, RGB or HSV as description system is irrelevant (all have three dimensions), as they are mathematically equally valid. One can be converted into every other.

The paper below contains example photographs showing how the reconstruction of three-filter photos leads to much better results if the Martian sky spectrum is included in the calculation:

http://marswatch.astro.cornell.edu/Bell_etal_SkyColor_06.pdf

The Cornell university has several interesting PDFs about the Pancam:

http://pancam.astro.cornell.edu/pancam_instrument/publications.html

The rendered object resembles the well-known MacBeth Color Checker, a calibration target widely used in photography.
Here are “real life” photographs:

http://www.ae5d.com/macbeth.html

Here’s a full spectral rendering of the chart. The illuminant is the standard daylight at 6500K color temperature, resembling average noon daylight:

http://s47.photobucket.com/albums/f188/albdo-co/?action=view&current=MacBeth_D65.jpg

And this is an example of choosing the wrong white point. In this case the illumination is D55 (like afternoon daylight) but with a white point resembling the D65 standard. This is a typical problem all photographers know:

http://s47.photobucket.com/albums/f188/albdo-co/?action=view&current=MacBeth_D50_non-adapted.jpg

And this is how the color chart would look like through the L7 filter of the Pancam, assuming D65 daylight again. As the glass interference filters are very narrow the resulting “color” can’t be rendered correctly on a computer monitor – the color is outside the gamut of the monitor. Nevertheless, this gives a good impression of the general appearance of the colors:

http://s47.photobucket.com/albums/f188/albdo-co/?action=view&current=MacBeth_L7.jpg

Furthermore it is not necessary to use the full filter set to reconstruct the sky spectrum to a high degree.
There’s a paper, “Chromaticity of the Martian sky as observed by the Mars Exploration Rover Pancam instruments”, that describes in detail how the color reconstruction process works and how the spectrum of the Martian sky is composed/reconstructed.
It also explains how it is possible to reconstruct the full sky spectrum to a high degree (with errors as low as 1%) even when using only the three filters L4, L5 and L6.
The Martian sky spectrum has its peak at around 610 nm wavelength, falling off strongly towards both the long and short wavelength ends of the visible spectrum, quite different from the sky spectra on Earth. The Pancam results (from the Horizon and Sky Flat Surveys) confirm the findings of the old Viking mission and not so old Pathfinder mission. Besides, the Luminance of the Martian daytime sky is very low compared to Earth’s sky at daytime.
Calculations including dust and Rayleigh scattering show that the average color of the Martian sky is very close to “dark yellowish brown” according to the NBS-ISCC color naming convention.
If the dust in the Martian atmosphere would settle completely (which has not been observed yet) then the Martian sky would look bluish-black to black at daytime (using NBS-ISCC nomenclature). A complete dust-free atmosphere would have an optical depth (tau) near 0.0 (optical depth is a measure of transparency, and is defined as the negative logarithm of the scattered or absorbed portion of light).
The optical depths observed by Spirit and Opportunity range between 1.7 and 0.2, changing with seasonal dust storms etc.)
Below tau=0.2 the color of the sky starts to change considerably because less light is diffused by dust particles that brighten the sky. So the sky gets very dark and finally, if there’s almost no dust anymore the hue of the sky changes to a very dark blue.
The Luminance of the dust-free sky is as low as 0.005, compared to 1.0 which stands for a Lambertian diffusor, meaning a material reflecting all light perfectly. The average Luminance of the average Martian sky (tau = 1.0) is only 0.17, still very low compared to Earth.

That’s not very surprising because the Martian atmosphere is very! thin compared to Earth, what people seem to forget all the time.

Again, you can calculate this with the same methods that are successfully used to calculate the atmospheric appearance of Earth’s atmosphere (using Rayleigh scattering to calculate the light scattering on molecules).

“but the dirt is multiple colors, and during duststorms, the planet can turn red, or orange”

should read:

“but the dirt is multiple colors, and during duststorms, the planet’s atmosphere can turn red, or orange”

When I look at images of Venus, and images of the soviet’s surface photos, the skies match the color of Venus from space. The planet is yellowy-white, the skies are yellowy-white.

I’m sorry, but I’ve just checked every other true color image from the Hubble Space Telescope, and the halo around Mars when its closer to the sun scatters light identically to the atmosphere on Earth. I’ve seen the illustrations of the “Red Planet” with it’s red skies, and the whole place being just red red red, and I’m sorry, but the dirt is multiple colors, and during duststorms, the planet can turn red, or orange, and even green sometimes, but on cloudless days its blue, because it projects a blue scatter of light from space. The closer Mars is to the Sun in its orbit, the bluer the skies.

Correct the Hubble imagers if they’re simply so wrong in their color calibrations, because according to some, every time their imaging mars close to the Sun they’re screwing the colors up.

Get real.

http://www.donaldedavis.com/2008%20new/CLRMARS.html