La Puce wrote:

I think it's too late to regain the colour.

I've checked some photos I took when the decking was originally laid.
The change in colour I've observed is definitely back to the original
colour, not just darkening of the grey. If the temporary change back
to the original colour is something to do with the wood fibres
temporarily retaining the water, then maybe the answer lies in finding
a treatment which will enhance the water-retaining properties of the
fibres.

Ken