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Hi,
Yes, an ever interesting question.
As far as we know the speed of light is everywhere the same in a vacuum. That is an important proviso. Light certainly does slow down in water, glass, air and a housebrick etc ;=]]
The various colours we see are all different wavelengths of light (but all still the same speed in a vacuum) and these may be separated out when passing at an angle through materials, such as glass.
Newton was not the first to see the spectrum of visible colours, that was Roger Bacon, who saw them in a glass of water (not gin and tonic). In effect he saw his own little rainbow.
Both observed that different wavelengths of light slow down differently in a dense medium. This is not good for us with telescopes, because it means that the colours can all come to a focus at different points, producing the fault called achromatism.
With filters the makers aim to exclude certain wavelengths such as neon or sodium emissions, and allow others through such as HA to enhance our view. These filters are flat plane and not designed to produce any spectrum, only block undesirable colours.
However yes all that is still part of the spectrum.
Cheers
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