I did 3 builders sand, 1 sharp sand, 1 cement and 1 lime to get this colour around my flints. I didn't want "white" that would contrast too much with the mortar which was 4 builders sand to 1 cement.
I actually meant making the whole thing a uniform colour but up to you. Any cut ends of the deck board need treating anyway. If you have any of the No Nonsense stuff put that on.
Aldi/Lidl sometimes do cheap deck treatments.
If you wanted to tone down those concrete pads, you could mix up some waterproof tile adhesive and cover it in the same gravel as the drive. Or get some offcuts of AstroTurf or paint with black masonary paint etc.