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.
Although he and his wife live in New York, they make frequent trips to Europe. With the iPhone, he can literally paint on the road, he said, sometimes capturing landscapes from the seat of his car.
"I like to have my studio in my pocket," he told ABCNews.com.
An artist who has always chosen the outdoors over the studio, Colombo said when people see him with a paintbrush or pen they peek over his shoulder.
But with the iPhone, "they don't know he's an artist," said Mouly.
That anonymity and convenience infuse his images with a distinctive freshness, she said.
"That's one of the reasons why I was so into his work for The New Yorker cover," Mouly told ABCNews.com. "It uses the technology to keep it free and spontaneous. I think you can feel it in the drawing -- it's done on the spot."
Depending on the subject and his intent, Colombo said he can complete a piece in 30 to 60 minutes.