I'm simulating a page being turned in a book. I'm using After Effects. An illustrator file is imported into After Effects and that file contains a few layers. One layer represents a page and another layer represents the rest of the book. The problem I have faced is that when the page/layer is turned the "back" of the page/layer isn't opaque. Is there a way to make the back side of a layer in Illustrator plain white or black?
Like all digital images, Illustrator files only have one side. There is no "back" to any layer or file.
Illustrator files have transparent backgrounds. Even though, by default, they appear white in Illustrator. You can turn on the Transparency Grid in Illustrator (View menu) to see the actual transparency.
If you want artboards/layers to be opaque rather than transparent, you need to add a filled rectangle the size of the artboard/layer.
Be aware that if using the "Simulate Paper" option in Illustrator's document setup a white rectangle won't do any good. One needs to turn off "Simulate Paper" option and use a rectangle at the bottom of the layer stack for any color/texture desired.