Our sky is blue. Would a planet that has 1% of earth's atmospheric density have the same colour sky assuming the sun is exactly the same as ours? If not then which colour would the sky be and why?
Could you determine which elements the atmosphere is made of just by looking at the colour of the sky?
Is it possible to know the density of a planet by knowing the colour of its sky?
Thanks in advance for the brilliant mind that sees this and answers my questions.
 A: If there is not enough atmosphere or stuff in the atmosphere to scatter incoming light, it will appear blac or very dark.. Think about how you can't see a laser beam middle mid-air unless it bounces off something like fog to
Our sky appears blue because blue wavelengths are small relative to the air molecules so bounce off them and get scattered more than longer wavelengths. At sunset the sky looks red because the sun being on the horizon causes sunlight has to travel through more atmosphere. So much blue light is scattered along the way that there is very little left by the time it reaches your eye. Longer wavelengths like red are larger than the air molecules don't scatter as much and so can reach your eye. All this is called Rayleigh scattering.
The particles in clouds are huge relative to visible wavelengths so scatter the wavelengths more evenly and so appear white. This is Mie scattering.
And this is why long-infrared (not near-IR) can penetrate clouds (think satellites).
A: The sky would look as if you were at a height of near 26 km, because that is where the pressure has dropped to 0.01 atmosphere.
A: The color of the sky doesn't just depend on the atmospheric density, but also on aerosols and particulate matter. Mars has about 1% of Earth's atmospheric density, just as you specify in the question. But its sky looks pale orange during the day, whereas sunsets look bluish. So that's almost opposite to what we see on Earth. The reason for these colors is scattering mostly by fine-grained dust particles. A nice picture is linked here (NASA).
The more general question is if you can tell a planet's composition by looking at its atmosphere. But from the context it appears that you imagine standing on the planet and actually looking. In that case, you can tell from the example of our own planet that this is not enough to deduce the composition of the planet. On the other hand, spectroscopic studies of the atmosphere do indeed help pin down its composition, especially if combined with other measurements. Most importantly, you need a way to deduce the planet's mass. This can be done if you find some other object gravitationally interacting with the planet (either a moon, or a spacecraft, etc.).
A: Why not make it simple? Let's say, that here on Earth nothing else changes, just that the atmospheric density decreases to your required level (even if it is impossible for the sake of argument let's say that). What do you see when you look up? Good old blue. Why?
The answer is Rayleigh scattering. If nothing else changes, this is the dominant form of scattering when the particles (atmospheric molecules) are much smaller then the wavelength of (visible 400-700nm) light. This is why the sky is blue now, and atmospheric density alone will not change the color itself.

After all, you have relatively small thickness near zenith, which makes most of the light scattered to you not too extincted due to Beer-Lambert law, while near the horizon the thickness is much larger, and the light scattered into the observer, in addition to becoming bluer due to Rayleigh scattering depending on wavelength, becomes also redder due to extinction along this long path. The combination of this bluing and reddening effects gives a color closer to white (which you can see in the daytime simulation above), or reddish-orange (in the twilight).

Why is the sky *uniformly* blue?
Will it change the shade? I bet it will somewhat, maybe it will be a different shade of blue.
