The Van Gogh Museum in Amsterdam unveiled on Thursday its newest addition: an 1882 watercolour of a pollard willow from Vincent van Gogh's early Dutch period. (Van Gogh Museum/Associated Press)

A young Vincent van Gogh was so struck by a dead willow leaning "lonely and melancholy" over a pond near The Hague that he knew at once he had to paint it.

"I'm going to attack it tomorrow morning," he wrote to his brother Theo on July 26, 1882.

The Van Gogh Museum unveiled the painting Thursday, the first addition in five years to its world-famous collection of works by the postimpressionist master.

At a time when the artist was still honing his skills in perspective, anatomy and proportion using pen and pencil sketches, the watercolor was a bolt from the blue, although its muted tones are still a far cry from the exuberant and colourful oil paintings that characterized van Gogh's later works.

"It's a very elaborate, well done watercolour and that's quite extraordinary in this period of van Gogh's oeuvre," said Marije Vellekoop, the museum's curator of prints and drawings.

"Out of the blue, in the summer, in July, he makes a series of watercolours ... with a lot of detail, but also very painterly, fluent."

The willow trunk droops over the water and a path wends its way to the horizon, where a windmill stands near a railroad depot.

'We didn't have anything like that in our collection'—Axel Rueger, museum director

Not unusually for a Dutch summer, gray clouds dominate the sky, but van Gogh also captured the occasional splash of deep blue as the clouds broke. The sky was almost identical Thursday morning — low gray clouds scudding over the landmark Amsterdam museum — as director Axel Rueger revealed the painting to the media.

Rueger said the painting, bought at auction in London earlier this year for € 1.5 million ($1.9 million Cdn), filled a gap in the museum's collection of van Gogh works.

"What's so special is it is for the first time a rather substantial work that he executes in colour," Rueger told The Associated Press.

"It comes from a very small group of works he makes at the time and we didn't have anything like that in our collection."

For now, it will hang at the Van Gogh Museum. Later this year it and dozens of other paintings will be shifted across the Amstel River to the Hermitage Amsterdam while the museum closes for several months for renovations.

Van Gogh wrote enthusiastically to Theo a few days after completing the painting, and included a sketch. The letter, on faded brown paper, hangs next to the completed painting in the museum. In it, van Gogh says he considers the willow the best of a series of watercolours he painted that summer.

"I think he was very happy with the result and he was also confident that he could also work with colour," said Vellekoop.