No, seriously. Having colors would enable people to abuse them. Of course, we also have a possibility for people to abuse headlines or subscripts, but colors can be very distracting if not used correctly.
In order to highlight stuff, we already have boldface and emphasized text. If you can't make clear what the error is, for example, you probably need to rewrite a bit.
I have nothing to add. Well, actually I do.
Based on my experiences with review queues on other Stack Exchange sites, people almost always either don't use any formatting at all - including line breaks - or they overuse formatting.
In forums where there's bold, emphasize, colors and larger fonts available, formatting tends to escalate: first you have long post, then you add some (well-thought) bolding, but after editing you think you should highlight yet another thing, but it boldface is not enough anymore, so you add another color. I'm glad majority of the people have not noted that there's actually support for large-font titles in SE.
In my opinion, even boldface and emphasize is rarely necessary on well-formatted posts (however, when iterating through list of requirements on the answer, highlighting either answer or quoted parts improve readability).
Using too much colors cause unnecessary problems for colorblind, and on low-end devices.
Instead of adding colors ("Yes"/"No"/"Partial"), maybe those points should be categorized, like this:
- Must do Y: I have used feature Y extensively. The only problem is with stability, when used with feature Z. However, there's rarely need to use these together, so it is not an issue.
- Must do M: my comments on feature M
- Should do X: [this plugin] might help with X, but it is not completed yet.
This does not require adding more formatting options, but makes it easy to see what features are there and what is missing. Of course, this is only applicable for e.g 5 or more bullet points.
This being said, I agree having easy-to-use icons available would be a good thing, something like ones in @David's answer. That's way better than having colored text, and on certain scenarios improve readability more than above suggestion.