1. It improves the presentation of information. The vast majority of colorblind can still see color. In this respect, we are no different from people who have normal color vision.

2. It has no impact upon the presentation of information. In these cases, the colors may not be distinguishable but there are other cues that convey information. I typically think of these cues as context. An example of context relevant to to exa is the column or character used to represent the information.

3. It degrades the information being presented. In addition to there being different types of colorblindness, each type seems to be variable. That is to say that most colorblind persons aren't actually blind to a particular color, they are simply less sensitive to that color. This means that they may be able to see a color if it is intense. Using colored text is really bad in this respect since it is typically less intense, likely due to a small number of pixels being illuminated. This reduces the legibility of text to varying degrees.

4. It makes information illegible. This is essentially the extreme version of case 3. This happens when the contrast is so low that color is the main distinguishing factor, so it may be nearly impossible to distinguish two adjacent colors (such as foreground and background colors for text).

Take anything that I say at face value. It is largely based upon personal experience. The only thing that I can say with certainty is that colorblind people see color differently. I suspect that people with normal color vision suffer from similar issues to colorblind individuals. It simply manifests itself in a standard way so that there are standard design methods to avoid it. Of course colorblind people throw a wrench into the works because their vision is calibrate in different ways due to their sensitivity to each color.