Hi Emma,
That silvery grey colour is the the natural weathered look all wood eventually gets to. Some people like that.
If you want to restore the origonal colour there are various products on the market which will do that. You could try Cuprinol decking reatorer. You simply paint this on, wait then scrub off. Idealy, then use a preassure washer to finish off.
This will reveal something of the origonal colour. To maintain that colour you might then apply a decking oil, which will colour and protect the newly exposed timber.
http://www.cuprinol.co.uk/products/deck ... rview.html