This is a good project to get you started working with the DOM. In this John Smilga tutorial, you will learn how to create a random background color changer. Many of the screenshots below are from here. If you have not learned JavaScript fundamentals, then I would suggest watching this course before proceeding with the projects.
You can click on any of the projects listed below to jump to that section of the article. That will test if you have really learned the concepts or not. Experiment with adding new features or using different methods. My advice for tutorials would be to watch the video, build the project, break it apart and rebuild it your own way. I have created a list of 40 beginner friendly project tutorials in Vanilla JavaScript, React, and TypeScript. The best way to learn a new programming language is to build projects.