Use a paintbrush to highlight the parts that you want to color.
Most apps take the same approach to this effect:
You can search for any of these terms in the App Store or Google Play Store to find apps that do what you want. There are many different names for the same effect: color pop, color burst, color splash, desaturate all colors except one, and selective colorization. Here are the best apps and tutorials that show you how to recreate this effect. You might have seen photos that feature this dramatic effect: one object in color with the rest in black and white.
