I agree that a style feature similar to what editors like Word offer would quite a lot influence the using experience and should therefore be thought through really carefully.
But highlighting colors...it's basically the same as different font colors. The same in terms of UX, implementation and even how the UI would have to look like.
Evernote is a big question mark for me. I understand that prioritizing is key in development and I understand that they can't implement all community requested features. I also understand that some features are much more complicated to implement from a technical perspective as it seems from outside. I understand that some features might destroy the carefully designed user experience behind the software. But adding different highlighting colors? Given the fact that it's one of the most requested features by the community? As I said, there is just no excuse in my opinion.