The new Color Pop feature that was highlighted this past week at Google I/O in Google Photos is now starting to roll out to users. The feature takes an image in your photos library that has a bright colored subject and then turns the background black & white.
The feature, at least right now, is 100% driving by Google Assistant, the AI built into Google Photos. That means you can’t force the issue. If the AI finds a photo in your library that meets its criteria, it will provide you the photo in the Assistant tab where you can save it to your library.