Da Vinci's "Salvator Mundi" was sold for $450.3 million in 2017 to the Abu Dhabi branch of the Louvre, but has not gone on display since then. The exhibition in Paris features a version of the same painting made by one of da Vinci's disciples.
The Mona Lisa has been behind safety glass since the early 1950s, when it was damaged by a visitor who poured acid on it. Since then there have been several other unsuccessful attempts at vandalising the painting.
Australian researchers looked for the most repulsive colour to help save lives.
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