If you like highlighting meaningful passages in traditional paper notebooks, you can use the highlighter tool in OneNote Online in a similar way.
Select the text you want to highlight.
On the Home tab, tap or click the arrow next to the Text Highlight Color button, and then click the highlight color you want.
To remove highlighting, tap or click No Color.
If you don’t want something highlighted anymore (or if you want to stop automatic highlighting so that you can type with plain text) you can remove the highlighting.
Select the highlighted text or multiple paragraphs with a combination of colors.
On the Home tab, click the arrow next to the Text Highlight Color button and click No Color.
Tip: To quickly remove all formatting (such as bold, underline, italics, color, superscript, subscript, and more) tap or click the Clear Formatting button next to the Bullets and Numbering icons on the Home tab.