How do we know the sky's not green and that we're all colour-blind?
90653
Because psychologists have studied the specific neurons that fire in the brain when the color blue is present.  We can decipher from the data that everyone is not colorblind.
953737
The answer to part one is "green", and the answer to the second part is "green".\n\nThat is why the sky actually appears blue at noon (blue light is more scattered) and red at sunset (red light is less scattered).
1089706
When transmitted light such as sunlight enters our atmosphere it collides with the oxygen and nitrogen atoms. The color with the shorter wavelength is scattered more by this collision. Because violet and blue are the shortest wavelengths the sky appears to be violet / blue. But because our eyes are more sensitive to blue light than they are violet light, we perceive the sky as blue.\n\nIt is because blue light from the sun strikes the air molecules and scatters and our eyes perceive it as blue.\n\nBlue in sunlight collides with air molecules and our eyes see it as blue.
185884
Neither is blue. \n\nThe SKY\nThe blue color of the sky is due to Rayleigh scattering. As light moves through the atmosphere, most of the longer wavelengths pass straight through. Little of the red, orange and yellow light is affected by the air. \n\nHowever, much of the shorter wavelength light is absorbed by the gas molecules. The absorbed blue light is then radiated in different directions. It gets scattered all around the sky. Whichever direction you look, some of this scattered blue light reaches you. Since you see the blue light from everywhere overhead, the sky looks blue. \n\nThe Sea.\nParticles and solutes can absorb light, as in tea or coffee. Green algae in rivers and streams often lend a blue-green color. The red sea has occasional blooms of red Trichodesmium erythraeum algae. \nParticles in water can scatter light. The Colorado river is often muddy red because of suspended reddish silt in the water. Some mountain lakes and streams with finely gound rock, such as glacial flour, are tourquise. Light scattering by suspended matter is required in order that the blue light produced by water's absorption can return to the surface and be observed. Such scattering can also shift the spectrum of the emerging photons toward the green, a color often seen when water laden with suspended particles is observed. \nThe surface of seas and lakes often reflect blue skylight, making them appear bluer. [[[ Montana reflection.]]] The relative contribution of reflected skylight and the light scattered back from the depths is strongly dependent on observation angle.
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(m)\n\nAs the sun begins to set, the light must travel farther through the atmosphere before it gets to you. More of the light is reflected and scattered. As less reaches you directly, the sun appears less bright. The color of the sun itself appears to change, first to orange and then to red. This is because even more of the short wavelength blues and greens are now scattered. Only the longer wavelengths are left in the direct beam that reaches your eyes\n\nThe sky around the setting sun may take on many colors. The most spectacular shows occur when the air contains many small particles of dust or water. These particles reflect light in all directions. Then, as some of the light heads towards you, different amounts of the shorter wavelength colors are scattered out. You see the longer wavelengths, and the sky appears red, pink or orange
1113645
A clear cloudless day-time sky is blue because molecules in the air scatter blue light from the sun more than they scatter red light.  When we look towards the sun at sunset, we see red and orange colours because the blue light has been scattered out and away from the line of sight.\n\n\nBecause the universe is not infinitely old, that puts a limit to how long the\nlight from all those stars can have travelled. So some of the light has not\ngotten here yet. Because stars do die out, that means that their light dies\nout too - which means that we may get the light from some stars at one time,\nand the light from some other stars at a different time, but not all the \nlight\nof all the stars that everywhere put together. Finally, because the \nuniverse is\nexpanding, the light gets weaker before it gets to us. All in all, this means\nthat there is not enough light from all those stars to make any point in \nspace\nbright.
569603
u can only see white when u look at the stars because u are seeing the combination of all the visible radiation. U can clearly make the colors through a telescope with a spectroscope attached. That is how they can tell the chemical composition of stars. However there are a few stars in the sky whose colors are plainly visible. (Betelgeuse in Orion the hunter.) but this has to do with the stars temperature. red for cool stars and blue for very hot stars.
508428
The same reason that your carpet isn't or your tee shirt isn't. The only light we see anything by is sunlight, but the light of the sun is reflected back in different colours according to the material reflecting it; in the case of the moon, grey dust.
371401
The sky has no color. The color that you see depends on the reflections from the ocean and other bodies of water and the suns ability to cause a reflection on the water.
7427
You take a test at the eye doctor.  Color blind people generally only lack the ability to see certain shades of a color like purple for example.  They wouldn't be able to differnciate between a sheet of blue and a sheet of purple paper.
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Different animals see different ranges of color.  Scientists can determine this largely by looking at the structure of the eye, to understand what light waves a structure responds to.
276568
You may be confusing light with pigment.  If there is no light, the light can't be colored.  However, the pigment of the grass which absorbs all but green light is the same, whether light is shining on it or not.
572848
Wow, what a prick...he probably knows less science than you do. He's definitely further off from the truth!\n\nColors as we see them are nothing more than photons. Which is electromagnetic radiation. We perceive different wavelengths (or frequencies) as different colors. So in a way colors do have a frequency associated with them. Blind people cannot recognize these though. The only way humans can recognize electromagnetic radiation (in the visible zone) is by seeing. By definition a blind person can't do that.\n\n(Well, technically we can also feel photons if lots of them hit us...as heat. But we cannot differentiate different frequencies, and the intensity of the light has to very extreme, like sunlight).
4502
There is a test where there are a bunch small circles within a larger circle. A number is written in the small circle with a different color. If you can't read the number then you are color blind. It's called the Ishihara test.
854178
Uh, I'm not smart on this, so I'm guessing here, but...\nSince different frequencies and wavelengths are what determines what color is being reflected, there has to be some way to quantify the amount of each in different hues of different colors.  Does that even make sense?  I don't know...
544792
Absolutely.  What I see and understand to be green might not be at all the same color you see and understand to be green, but we both recognize it in the same objects and both agree to the name of the color. Ther is no way to tell really whether what I see is what you see.
347339
Very interesting question. Yeah, they perhaps have no CONCEPT of how things looks like...say color. How would they know what green looks like if they have never seen green. Shapes and sizes perhaps they can feel with their hands, and imagine it. Or they can try to understand other things, through language. But, for the most part, when they actually SEE things, I bet it'd be a whole new world for them! They'd be astonished I guess :)