It's very likely that the only difference between them is that one was on top of the stack in a timber yard so has faded from sunlight and rain, the rest haven't... yet.
Just doing nothing is probably by far the best option. The rest will go the same way if you give them chance, they just haven't seen the sun yet.
Anything you do to this one plank will be the only difference between them in a couple of months, you'll then end up colouring up the rest, then they'll be different in a different way bla bla etc.
If it really really bothers you, to the point that you're worried your dinner party guests may recoil in horror with distraught expressions, then treating the whole lot with something should be the one and only option. They won't match instantly but will be less different. Over time, they'll all eventually match.
But treating the whole lot with something is probably a very good idea anyway. Buy a big brush and some kneepads, get something good on the radio and off you go.
Or just pour a beer, relax and enjoy your new decking.