Some pages use the Enter key to scroll down pages that are longer than the screen view. The Enter key may also lead to the next logical page or back you out of a page that is the end of something. You can navigate by using the Toolbar buttons and the regular links too, of course. The Web Page Toolbar at the top of each page has colored buttons that take you to different pages or views. The others will vary depending on the page's content.
Yellow color is there for visibility.
Grey color means you are already in the page the button normally leads to. Now it won't go anywhere. View of the page on the monitor can be changed . A low setting gives large letter sizes over a short width and a high setting gives small letter sizes over a large width. The high setting shows the least cluttered view with the most open space.