The bookmark icon is available in two colors - white (default) and blue (alternate). Change the color of this icon if you would like to highlight the bookmark for any reason.
For example, it is useful to distinguish between words you have already studied and those you have not yet studied.
Firstly, tap the Bookmark icon on the tab bar and then on the bookmark you wish to edit.
Tap the Edit icon on the top-right tool bar to open the Edit View.
Tap the Bookmark icon on the top right to change the icon color to blue.
To reset the icon color back to white, just tap it again.