Does O2 have a color in the gas phase
Background

$\ce{O2}$ exists as a paramagnetic, triplet since the two electrons in its two (degenerate) HOMO orbitals are unpaired. There are 6 known phases of solid oxygen with color ranging from pale blue to red to black.  In the liquid phase it has a light blue color.  This color is due to light absorption by the ground state triplet according to the following equation
$$\ce{2 O2(^3\Sigma_{g}) -&gt;C[{h\nu}]\   2 O2(^1\Delta_{g})}$$
This absorption requires light from the red region of the spectrum (~630 nm).  If red light is absorbed , then blue light is transmitted or reflected and this gives rise to the blue color associated with liquid oxygen.

Answer

Note that the light absorption requires 2 molecules of $\ce{O2}$ and a photon, a 3-body process. If we now consider the probability of this 3-body process occurring in the gas phase, we can see that, since a gas is much more dilute than a liquid, the probability for of these 3 items coming together at the same time will be much smaller.  Hence, the probability of photon absorption in the gas phase is much reduced.  To the human eye oxygen gas will appear colorless (at normal and reduced pressures).  However, if you place the gas in a cell and record its visible spectrum you will still be able to detect this absorption.