Highlight Tool is a Google Docs Add-on for educators and students. It's a tool that enables users to more efficiently highlight their Google Documents and keep track of what each color means.

There are a lot more features such as sharing highlighter sets with other users, maintaining multiple sets of highlighters for different use cases, extracting highlighted text from a document, and much more.

The github pages for this repository describes it in a lot more detail with images and a preview of the add-on.