No matter what you do, the timbers will 'grey' to a certain degree. Wood cannot keep its new color forever. The only way to stall it is to put some kind of UV-protection over it, and personally I'd rather it be grey. Granted, this will not be the same as leaving them in full sun, but it still is something to note. UV, water, and a slew of other issues will fade the color and otherwise change it.
For oak, they'll dull down quite a bit after a few years, they'll never stay as bright as newly exposed wood. But again, I still agree planing down to expose fresh wood is better than leaving them, for a number of reasons.