The famous Van Meegeren forgery "Christ and the Adulteress", which was sold to Goering, used a more modern blue instead of the original Vermeer's ultramarine - and for a good reason, because Van Meegeren could not, during wartime, obtain ultramarine from London, which is where the only contemporary vendor was located.

It could be detected by contemporary means, but no one thought about it in time, and there was no wisdom-of-the-crowds yet.