[teamonkey]

You can use a regular bayer-matrix colour camera (or colour film) for astrophotography, it’s very common in amateur circles.

However, mono cameras and filters are also often used because the interesting colours are often not R,G,B of the visible spectrum but certain emission frequencies of hydrogen, silicon, oxygen and others. These are often combined to make false-colour images.

Filters also help penetrate light pollution by keeping only the frequencies you’re interested in and rejecting much of the light reflected by the atmosphere.

In addition, getting lenses to focus all frequencies of light equally well at the same time is quite tricky. For a mono camera with narrowband filters, this is less of a problem: you can focus precisely on one colour at a time and you can get away with simpler optics or cheaper glass.