The app is called ColorSplash. In a nutshell, ColorSplash allows you to shoot a photo—or use one from your photo library—and apply a greyscale layer to it. You can then swipe your finger over parts of the image to reveal the original colour, thereby creating a black-and-white photo that colourizes areas that you want your audience to focus on.
You can reveal as much or as little of the original colour as you like. For example*,
I must have had Siena on the brain because it's been two years this week since we were there.
This photo of the Siena Duomo was originally shot with my D-SLR. I imported it into ColorSplash, which rendered it as a greyscale image. I then swiped over the glass window to reveal the original colour.
It was super-easy.
For extra effect, I took this new image and imported it into the Instagram app, cropped the photo, applied another filter, and voila:
I can see myself having all kinds of fun with this app. So, I raise my glass to ColorSplash. And here's proof:
Okay, the glass isn't raised. But I'll raise a cheer nevertheless!
* Although ColorSplash is an app for the iPhone, all of these photos were originally shot on a Nikon D80.