First off, it is impossible to see "new" colors in a dream. See O'nus's post in the tutorial section, or do a search. I remember making a couple long posts about this a couple years ago.

Secondly, if you could see other forms of electromagnetic radiation, it would NOT necessarily manifest itself in the visible radiation part of the spectrum. It might have a "color" to it, but it would most likely have a unique attribute. It seems people seem to think it would simply look like a color because that is all we can comprehend, and when we take pictures that can detect other forms of electromagnetic radiation, we see it manifested in wavelengths we can see. It is impossible for our minds to comprehend what another form of electromagnetic radiation wold actually look like, simply because we have never seen it before. It is the same way with imagining a "new" color. It is absolutely impossible to imagine a "new" color in our brain. Perhaps another form of EMR would be a color, but it would be a color we've never seen before. We cannot comprehend what it would look like though.