One common molecule that has an emission spectrum is nitrogen. It emits spectral lines in the red and blue part of the spectrum and is responsible for the red seen in energetic auroral displays. So how come nitrogen doesn't absorb red and blue in the air, leaving the air with a greenish tint? Because the nitrogen molecules have to be pumped up to fairly high energies first. They have to be excited by ultraviolet light or energetic particles, then they give back some of that energy as visible light. An analogy might help here. It's not the casual weekend gamblers in Las Vegas who give $100 tips. It's the high rollers. Casual gamblers just don't have enough money, but the high rollers won't miss it. Ordinary visible light and atomic collisions aren't energetic enough to get nitrogen molecules to levels where they can emit visible light. The molecules have to be at much higher energies before they can give back any energy as visible light.