Why are gasses difficult to see? Why are gases difficult to see?
 A: Chemical factors
The more "localized" the electrons are the higher frequencies they naturally vibrate at (like a shorter guitar string playing a higher note). Gases must be simple, small molecules otherwise they would condense. Small molecules can't have electrons that are delocalized over many atoms. All substances have tightly localized electrons that let them absorb deep ultraviolet and low-energy xray light. Most dyes, however, are complex molecules with delocalized electrons that allow longer-wavelength visible light absorption.
There are other ways that objects, including certain gases, can absorb visible light, but they are the exceptions rather than the rule.
Physical factors
Gases are 1000 times less dense.
Gases alone can't form heterogeneous structures. Water is clear but snow and foam are white because the refractive mismatch between ice/water and air scatters light in random directions. Any two gases when mixed (unless they react) will form a homogenous mixture.
A: Most obviously, gases contain not many atoms per cubic meter. There is a lot of space between them, and the photons will just pass unimpeded.
A second effect is that light, being an electro-magnetic wave is affected by electric fields and conductivity. Metals reflect light, which makes them obviously not transparent. This is directly because metals conduct electricity. Even for non-conductive solids, they're generally solid because all the atoms stick closely together by electro-magnetic forces (not gravity nor nuclear forces). In gases, the atoms don't stick together because the electro-magnetic forces are weaker, and so these forces also have less influence on photons.
A: Gases are transparent to light, just as glass is transparent. Some gases are coloured, such as chlorine which is greenish, and bromine vapour, which is brownish.
Which begs the question, why are some substances transparent to light. And that is a good question.
