The yellow pigment Van Gogh used in an iconic painting is fading to brown, new research finds.

A team of Dutch and Belgian scientists studying a Vincent van Gogh “Sunflower” painting discovered that the artist used two differing yellow pigments in the work, one of which is extremely sensitive to light and is fading to a brownish color over time as a result. The researchers spent two years painstakingly analyzing the painting—including through advanced x-ray technology—currently held by the Van Gogh Museum in Amsterdam. The museum is assessing the findings but has already lowered its galleries’ lighting in an attempt to slow the aging process, which is not yet visible to the naked eye.

The discovery is a major concern for the museum, which believes the light-sensitive shade of yellow was frequently used in the artist’s works. The pigment appears on its own and mixed in with other colors, such as in the flower’s green stems. Frederik Vanmeert, one of the researches on the team, told The Guardian it is “very difficult to say how long it would take for the change to be obvious, and it would depend a lot on the external factors.” Marije Vellekoop, the museum’s head of collection and research noted that they will thoroughly review the results of the analysis and continue to “pay further attention to discolouration.”