According to Orlando Figes, Chagall was invited to decorate the streets of Vitebsk in celebration of the first anniversary of the October Revolution.  Bemused local officials asked the obvious question:

‘Why is the cow green and why is the house flying through the sky, why?  What’s the connection with Marx and Engels?’

Source: quoted in Orlando Figes, Natasha’s Dance: A Cultural History of Russia, London: Penguin, 2003, p. 45