You can also move these layers to the top and make the opacity 100% in order to see what your pages will look like with the excess trimmed off or to see what the safe tone would look like.
The two greenish layer(s) are to act as reminders of where the spine will be. The darker colors is to simulate the idea of the paper being glued to the spine. The visibility of these layers can be turned on and off based on which one you want, but both are default to be visible. These two layers are merged as one in the splash/double version since there is no concern for it being a left or right page when both pages are visible in this particular version.
Single Page
Splash/Double