A Van Gogh watercolour that appears to show a midday scene of the French countryside under a dreary sky is something markedly more exciting, conservators at Tate Britain have discovered.

The true colour of Van Gogh’s sky and river in The Oise at Auvers is not a coffee-stain brown but a vivid pink, it was revealed on Monday before a blockbuster exhibition at the gallery in London.

The discovery that the paint had faded dramatically was made by conservators as they prepared for a show examining the artist’s relationship with Britain.

They removed the watercolour’s mount and frame and found evidence of the original colour scheme with the dazzling pink leaping out. A digital recreation has been made showing Van Gogh’s true intention.

“It is amazing, isn’t it,” said Carol Jacobi, the lead curator of the exhibition. “The thing that fascinates me is that he has got this particular effect you get at the end of the day when the sky is lighter than the landscape but it will light up in the water that’s in the landscape.”

‘… I have seen the whole sky coloured pink and bright orange.’ A digital reconstruction has been made showing the original colour of Van Gogh’s watercolour. Photograph: Joe Humphrys/Tate/PA

It was a natural effect that Van Gogh adored. He saw it as humanising the landscape and he particularly admired it in John Everett Millais’s painting Chill October, Jacobi said.

The paint has faded because Van Gogh, virtually penniless his entire life, used a cheap pigment of pink paint.

The Oise at Auvers was painted in May 1890, two months before he died after shooting himself, aged 37. That means it is one of his final painted landscapes set around sunset.

The discovery fits in with observations Van Gogh was making in letters from around the time, including one in which he wrote “… I have seen the whole sky coloured pink and bright orange.”

Jacobi said: “He was in Auvers only for a few months and he was creating a picture a day; he was incredibly productive in that period. There is an amazing energy in the late paintings, a joy of nature.”

The Tate show is the gallery’s first Van Gogh exhibition since a popular one in 1947.

More than 50 works have gone on display exploring how the artist was inspired by British art and literature throughout his career, and how he in turn inspired many British artists. It also tells the story of Van Gogh’s years as a young man living in London in 1873-76.

• Van Gogh and Britain is at Tate Britain 27 March until 11 August.