@LSTakoyaki There is nothing fatal about her condition (as far as we can tell), but since her colorblindness is gradually worsening, she makes a list of colorful things she wants to see before completely losing the ability to perceive colors. Calling it a bucket list is just her way to express how important it is for her to do so.
If you are unaware of what is going to happen in the series please refer back to Chapter 11 with the foreshadowing if you missed it, DO NOT DO IT IF YOU DON'T WANT TO KNOW PLEASE