Vincent Van Gogh described his now-famous bedroom paintings from Arles, France, to his brother as having lilac and purple walls. Weirdly, the walls in his three paintings currently look more blue than purple.

Conservators at the Art Institute of Chicago analyzed the paint recently and they think they have figured out why.

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Curators at the Art Institute of Chicago were preparing to show all three paintings, when they asked their colleagues to do an in-depth scientific analysis. They wanted to know the order the paintings were done, and why the color differed from Van Gogh’s own description, the American Association for the Advancement of Science reported.

Francesca Casadio, who co-directs the Institute’s Center for Scientific Studies in the Arts, and conservation microscopist Inge Fiedler took a microscopic sample from the bedroom painting in their own collection.

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Under the microscope, the paint surface looked mostly light blue with a sprinkling of darker pigments. But then Fiedler turned the sample over. ”That’s when she came screaming out of the lab saying, ‘It’s purple! It’s purple!’” Casadio explained in an AAAS video.

Then the scientists used a technology called macro-X-ray fluorescence or MA-XRF to identify the pigment in Van Gogh’s paint. Standard X-ray doesn’t work on organic elements, but MA-XRF scanning does.

Turns out that the paint was made with carmine lake, a pigment produced from crushed bugs. Over time, it faded down to blues. Working with a color scientist, the conservators created a visualization of the Van Gogh bedroom that brings the purple back. They presented the re-colorized version over the weekend at the AAAS Annual Meeting.

More analysis revealed that the troubled artist completed the painting normally shown at the Van Gogh Museum in Amsterdam first, the one usually displayed in Chicago second, and the one in the Musée d’Orsay’s collection last.

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But here’s the twist: “In all truth, the walls of the actual bedroom in Arles were white, were whitewashed” Casadio said in the AAAS video. “The purple is his own interpretation.”

Or maybe he really did see those hues, depending on the time of day. As we know from that infamous dress debate, optical illusions abound.

The Van Gogh’s Bedooms exhibit at the Art Institute of Chicago displays all three paintings in one place and runs through May 10. If you do go, just picture all the walls to be more purple than they appear now.