It seems a layer of varnish added later to protect the work is in fact turning the yellow to a greyish-orange colour.

BBC: Van Gogh's Flowers In A Blue Vase damage seen in X-rays

He painted directly onto this surface, often in acrylics, sometimes brushing on a layer of darkening varnish before the paint cured.

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The purpose was to determine not only what was in the samples in terms of atoms and molecules, but also the precise structures in the interface layer between the original paint and the varnish.

BBC: Van Gogh's Flowers In A Blue Vase damage seen in X-rays