I HAVE just returned from a glorious walk on Dartmoor in good weather, which meant we could see for miles around.    To the north and west what stuck out most prominently was a large number of wind turbines.   This is partly because they were moving but mostly because they are white.   It occurred to me and my companions that if they were painted black one would really hardly see them and the offence of their presence in the rural landscape would be enormously reduced.  Why are they painted white?   Has anyone ever thought of this? Jennifer Reynolds Bridestowe