It seems a layer of varnish added later to protect the work is in fact turning the yellow to a greyish-orange colour.
High-intensity X-ray studies described in Analytical Chemistry found compounds called oxalates were responsible.
But atoms from the original paint were also found in the varnish, which may therefore be left in place.
It is not the first time that the bright yellows that Van Gogh preferred have been examined with X-rays.
In 2011, an article in the same journal from a team led by the University of Antwerp’s Koen Janssens reported that a pigment Van Gogh favoured called chrome yellow degraded when other, chromium-containing pigments were present.
The new work was begun during a conservation treatment in 2009, when conservators found that the yellows in Flowers In A Blue Vase – this time from a pigment called cadmium yellow – had turned greyish and cracked.
Normally, cadmium yellow grows paler and less vibrant as it ages.
So the team again took tiny samples of the work to some of Europe’s largest sources of X-rays: the ESRF in France and Desy in Germany. Both use vast particle accelerators to speed up electrons, which spray out X-rays as they pass around the accelerators.
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.
That is where the team was shocked to find a compound called cadmium oxalate as the cause of the grey-orange pallor.