You can put solid colors, textures or animatics in your iMovie timeline for a variety of reasons. Use them to allow your audio to play when there is no video to show. Or, use them to plan your movie.
Related: Animated Backgrounds in iMovie Trailers, Why are iMovie Titles changing?, Merging iMovie Projects (iMovie ’11), Graphics with transparent backgrounds into iMovie, iMovie pic in pic.

Very helpful,thanks.
Great ideas on how to use backgrounds. I am a recent subscriber to your newsletter and I am getting good tips.