There are thousands of reasons that people might use colored text highlighting. Maybe millions. Maybe a different reason for each user.
Here's an article from Fast Company that explains "How Color Coded Notes Make You a More Efficient Thinker."
Here's another reason, and I think it's compelling:
People who have been asking for this for half a decade have concluded that you're not listening to them, so they've moved on to other platforms.
And after waiting five years, we get homework assignments from you: "Prove to us that you need it badly enough." Oy, vey!
Instead of "Refreshing the Evernote Brand," whatever that means, why not give your loyal users the features that have been asking for?