This setting lets you change the type of highlighting the site does.
The first three settings, Yellow/purple, Green/light red, and Blue/orange let you highlight text, but control which color you see. The first color you'll always see. The second color is the color you'll see when you select certain Display Modes.
If you have a color vision deficiency, try the different color choices to find what works best for you.
You can select Underline if you prefer not to see colors.
You can select None if you do not want the site to indicate to you which words were rewordified. You can still click (or tap) the hard words to learn what they mean, hear them, and learn them.