The green line represents the half-inch safety margin around each page.
We recommend keeping all important items (such as text and photos) within this guideline to ensure a well designed book, with no items too close to the trim edge. If candids or clipart are meant to be trimmed off for a design effect, it is okay to have their location extend outside of the green safety margins. However, it will still trigger a warning about being too close to the trim edge to make sure that you intentionally want it to extend beyond the margin.
The red line is the trim line—or where each page will be cut.
If you wish to extend an item such as a background or photo off the page, you need to extend the item beyond the red trim line until it touches the outside of the yellow border.
The yellow border is the “bleed” area of the page. The bleed area (yellow border) will be trimmed off during the production of your book. The purpose of the bleed area is to make sure that items you wish to bleed (extend beyond the trim line) in your book are extended far enough so there is no possibility of a white strip of blank paper on the edge of your page.