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Baz,
As you are aware, the colours of light have different wavelengths. Red light has a longer wavelength than blue light, which is more energetic. When light passes from a vacuum into the atmosphere it will slow down marginally but this won't break it up into the spectrum. To break the light up into the colours of the spectrum (dispersion) it must be bent (refracted) through a denser medium. Blue light bends more than red light because of its shorter wavelength. I don't think the atmosphere at higher angles would produce much dispersion. However, water droplets in the air can refract light to produce a rainbow (blue inside the bow, red outside the bow). A glass prism will also refract light, blue bending more than red. The sky appears redder at sunset because other colours are bent more out of our line of sight by particles in the atmosphere.
Through a telescope with poorer optics, you might see colour fringing around stars due to dispersion of light through the glass lenses. As Geoff stated, the different colours are refracted to focus at different points.
A hydrogen-alpha filter passes mainly the red light at the H-alpha wavelength of emission nebulae. A double ionised oxygen filter passes mainly the green light at the O III wavelength of diffuse nebulae and planetary nebulae. As Geoff stated, they are still part of the same light spectrum.
Regards, Rob
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