To be honest it is quite difficult to state if the base was unpainted or not. In many pictures it can be noticed that it wasn't. But there are images in which it looks like painted. Here is a part o f the well know shot of the Castle Bromwich factory "parking" with finished Mk.IXs I cut out the closest plane. IMHO either the base was painted or made of the material of a close colour tone to the Ocean Grey paint. The other kites standing there look similar.
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Basicly the dark tone of the base can be found though on the other Spits used .
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But it looks like the maintenace crews could overpainted it though ...
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the pic sourcer: the net.