With your smartphone, inexpensive software and a bit of cloth or paper, you can make your own “green screen” movies. No matter what you call it — chroma key, green screen or blue screen — it’s the film and video technique that gives your local TV weatherperson something in common with the Avengers movies: artificial backgrounds inserted behind the action. You simply record your subject in front of a solid green or blue screen, and then add a touch of software magic to change the background. Dozens of free or inexpensive apps allow you to use the technique on your own clips. It’s a great way to jazz up your presentations and other videos — or to keep children busy with a weekend project filming their own toys in action scenes. Here’s how to get started. First, you need a big piece of solid green or blue cloth or paper to use as a background when recording. You’ll also need a place to hang it and strong lighting aimed at the screen to keep shadows and fabric wrinkles from showing up as blotches in your video. You can get a few yards of cloth at a fabric store for less than $20, or buy a professional chroma-key backdrop, starting around $30, at a specialty store like B&H Photo ...