Among the tens of thousands of photographic materials attributed to Vivian Maier — prints, negatives, rolls of undeveloped film — are 40,000 or so Ektachrome color slides. Like the rest of the photographer's work, these were never meant to be shared with anyone; in her final years, Maier literally kept records of her artistic output in the dark, stashing them in a storage facility until a 2007 auction pulled the trove into the public eye, along with its late creator.

The black-and-white pictures taken by Maier — a highly private, self-taught artist who worked as a nanny in Chicago — are world-renowned portraits of city streets and those who walked them. Some include the photographer herself. 

But Maier's color images have been less visible. The reason for this is largely technical: The photographer left behind boxes of undeveloped Ektochrome positives, and according to curator Anne Moin, her color photographs have been the most challenging to recover, requiring the specialized skills of chemical developers.

Still, the work to develop some has been done, and about 150 are gathered in a new monograph. Vivian Maier: The Color Work (November 6, HarperCollins) represents the first book of the prolific photographer's color images, and its release coincides with an exhibition of corresponding prints at Howard Greenberg Gallery in New York City.

Many of these photographs have never been seen by the public before; even Maier might not have gone as far as to examine the processed negatives. As Colin Westerbeck, former curator of photography at the Art Institute, writes in the introduction, "It’s not certain that she ever opened the boxes and saw the results herself."

Maier's 40,000 color slides date to the last 30 years of her life, from the 1970s to the 1990s. Many were shot in Chicago and New York with a 35-millimeter camera, which replaced the twin-lens Rolleiflex Maier had favored while shooting in black and white.

Browsing the spectrum of streetscapes, it's apparent that she made the switch because she was thinking more about color. She captured conspicuous patterns, like a rainbow bouquet of balloons, but also vivid minutiae, like the flash of a cherry-red heel cheekily paired with a wan leg cast.

The medium opened new avenues of expression for the quiet but tireless observer. Indeed, as Joel Meyorowitz writes in the foreword to The Color Work, “Maier was an early poet of color photography."