What is the film that covers the tongue?
It isn't really a film. The tiny bumps that cover your tongue are called papillae, and are normally pink in color. However, they can become inflamed and white when irritated. The appearance of the white "coating" is caused by debris, bacteria and dead cells getting lodged between the papillae. 

You may be breathing through your mouth when you sleep, which is drying it out. Bacteria may also still be the cause; you may not be brushing well enough or your toothpaste may not be correctly doing its job.

The color variations may be due to different types or amounts of debris, or the color may vary with different conditions listed here. Tongue color changes also often occur with glossitis (inflammation of the tongue itself).