Why is the Sky Blue? No, Seriously

Okay, so basically, atoms are really small. They're so small that they're smaller than light -- light has a size over which it likes to interact and interfere called its wavelength, and thousands of atoms can fit side-by-side across the visible wavelengths. (Smaller light also has a little bit more kick to it per photon, which is why we physicists tend to think that cell phones cannot cause cancer.)

The smaller light is closer to the size of the atoms that it's scattering over and that's principally why it scatters more strongly. It's what we call elastic scattering; the light doesn't lose any energy and therefore keeps the same color, rather than becoming redder. So the light bounces off of the air without kicking it.

One curious fact that you might think about, which bugs me a little bit, is that the color of the sky is not isotropic. It really depends what direction you're looking. Imagine that you're facing the setting sun, and you look up, and the sky's still actually pretty blue, but over "next to the Sun" the sky is pretty orange. But now teleport yourself to the distant fields that this sky is right above, and you will be jumping closer to the Sun -- so the Sun will actually be higher, there will be less red in your sky, you look up and the sky also seems quite blue to you. Step back and look at the globe rotating from space, half black, half in light -- as sunset comes, you're rotating away from the Sun, into the black. The only isotropic way to get a red sky around the Sun is to have the horizon be a sort of red band of sky, but that would have to "wash over" the sky as we crossed over it. I mean, I'm sure that if you looked from space the objects would seem redder, because the Sun gets all coppery as it sets, but the color of a given box of air in our atmosphere is actually as far as I can tell different in different directions, and nobody ever seems to want to calculate this.

Point being, it's not like a blue wall, where the wall is glowing blue in all directions. The same sky glows in different colors in different directions.

As for your specific question, if you're right that methane is colorless (i.e. that it has no absorption frequencies in the visible spectrum) then that's certainly strange, yes. Are you sure it's not colorless in the sense that air is colorless -- that you need to look through some kilometers of air before you see some tiny-in-probability scattering event?

Some searching on DuckDuckGo reveals that there are a couple of different effects; one is that methane might form smoggy cloudy effects as on Titan and Venus which are not colorless; but that without those effects methane does indeed weakly absorb red. They claim that this might cause the sky to become blue-green under our sort of illumination:

http://boards.straightdope.com/sdmb/showthread.php?t=534194

The article could have mentioned that Saturn's moon Titan has (a) an atmosphere containing some methane, and (b) an orange appearance. But (b) is thought to be the result of organics in the atmosphere, not so much the methane (http://en.wikipedia.org/wiki/Titan_(moon)): "The atmosphere of Titan is largely composed of nitrogen; minor components lead to the formation of methane and ethane clouds and nitrogen-rich organic smog."

So it seems the orange color results from ordinary preferential reflection of certain wavelengths from smog particles (as in many U.S. cities), not Rayleigh scattering or another similar effect as in earth's atmosphere. As to the early earth's atmospheric color, I think that's a stretch, depending on things we just don't know.

Also, the Wikipedia article's use of "organic" should be understood to mean "containing carbon compounds", not necessarily organic in a broader sense.