If you saw him on the sidewalk, cradling an iPhone in his hand, you'd think he was texting a friend or e-mailing his wife.
But pull in a little closer and you'll see something else.
Lively street corners. Graceful silhouettes. New York City skyscrapers stretching past the clouds.
All captured by a fingertip dancing across an iPhone screen.
Using the Brushes application, one of thousands of available for the iPhone and iPod touch, he has digitally painted dozens of iconic New York scenes, including Grand Central Terminal, classic downtown delicatessens and the Empire State Building.
This week, one of his sketches will assume a spot coveted by artists in New York and all over the world: the cover of The New Yorker magazine.
"It's one of those street carts that sells hot dogs and pretzels," Colombo said. The cart he painted is in the heart of New York's Time Square, on 42nd Street, but, he added, "it's also every food cart in New York. It's a way to know you are here and not in San Francisco, Lisbon or Tokyo."
Despite the seemingly unconventional medium, Francoise Mouly, the art editor for The New Yorker, said the moment she saw Colombo's work she knew it was a fit for the magazine's cover.
"It's his point of view on what is happening in the city," she said. "He is showing you people in silhouette that are both generic and very specific at the same time."
Aptly named "Finger Painting," the image chosen for the magazine, she said, shows off "that mixture of dark shadows and slightly ominous feeling that can be in Times Square at night."
Colombo has worked with watercolor and pen for decades, he said, publishing illustrations in magazines such as The Village Voice, Mother Jones, Playboy and even The New Yorker once before in the early 1990s.
But the Lisbon, Portugal-born artist started experimenting with the iPhone in February, immediately drawn to the convenience of having all the tools he needs in the palm of his hand.
The key to a successful image, he said, is working from the background to the foreground, constantly layering on the colors.
"You have to do the stage before you do the characters," he said.
Although Colombo's work is among the more visible pieces of iPhone art, thousands of people around the world use the $4.99 Brushes application to create digital images.
Steve Sprang, the application's developer estimates that, since he launched Brushes in August 2008, about 40,000 people around the world have downloaded the program.
The Mountain View, Calif.-based developer worked for Apple for seven years before striking out on his own.
"I've always been interested in computer graphics," he said, adding that he dabbles in art a bit himself. "When I was thinking about what to do when I left Apple, it seemed like a natural fit for the device. You're touching it, it seemed like a natural step to want to draw on it."
The intention, he said, was to create an application that would appeal to everyone but he hoped that working artists would find it valuable.
He said he's been very pleased by the response, pointing out that about 500 iPhone artists have posted the images they've created with his program on a Flickr Web site.
Patricio Villarroel, a Paris-based artist, was also one of the first to download the Brushes application. Since 1996, he said he's been creating virtual paintings on his Mac. He migrated to the iPhone 2 weeks after Sprang launched Brushes.
But though professional artists have gravitated to the program, Colombo insists that iPhone art is for everyone.
"People should have fun," said Colombo. "It's good to do something that you can record your moments, create your own images. I still recommend people to play. I don't think play enough with pens and pencils and colors as much as they should."
ABC News' Wonbo Woo contributed to this report.