You have to think about: how do you define colour? It is the part of the spectrum which is reflected back to your eyes rather than absorbed by the object that it strikes. Glass has a very small reflection coefficient, so not much light is reflected back to you to begin with. As for the spectral variation of the reflection coefficient, I am not sure about it, but if I had time I could probably look it up. But I think your eyes won't see much difference if it reflects, say, 2% of the intensity of red light and 1% of the intensity of blue light. Both numbers are so small that you don't notice a particular colour being preferentially reflected. So the colour would be - whatever preferred word for no colour at all - I would call it "clear".
Now you say, but the prism splits light into a rainbow. Yes, but that is the light that goes through it, not the light that is reflected. If you're sticking to the reflection definition of colour, then the prism is clearly clear. 'laugh] You can't see the rainbow part unless you project it onto something, which isn't the same thing as the colour of the prism itself.
Polarization is a lot harder to explain in words (physics is all about drawing things, which I am terrible at), so I will leave that unless someone PMs me to ask.
