>Does the percentage indicate the ratio of the people in the casual/hardcore, female/male, and teen/YA/adult blocks that will buy your single?
Correct. If casual+male+teen = 30%, then it means that 30% of all casual male teen fans are going to buy the single. >100% means some fans will buy multiple copies.
Other things that increase sales - buzz and fame level of the single. But there's also a penalty if fame drops compared to the previous single.
Colors don't mean much when you're creating a single (it's just >50% = green, <0% = red), but they do matter when you release it. E.g if casual male teen fan's bar is red, it means that appeal of the single to them is lower than the previous single, and they're not satisfied with it.
Other than a bonus to sales, buzz is kind of a dead mechanic right now, I will either rework it or get rid of it. So you can just ignore it.