Sentence:  From his window on to the courtyard he watched the low, <WSD>gun-metal</WSD> sky over Châteauroux and the bare barrack trees, and was perversely reminded of the harsh desert light, the heat, the colours of the dunes at evening; the overcast skies. 
Meaning of the word marked with <WSD> tags:  A dark grey or bluish-grey colour; gunmetal grey.