As you can see, the yellow primer is much lighter than the yellow of the wall. Eventually all of the bookshelves will be that same darker yellow. Here's a close up to see the difference in color better:
After painting the primer on the shelves it was much easier to see all of the areas where I either forgot to putty, or where the putty job was simply sub par.
|Giant gap where my shelf meets its dado cut.
|Same spot filled with putty.