Stone is stone. There is no "stone color". Making it darker could result in it looking like concrete. Indeed, stone roads look lighter than concrete, and that's mainly what we want here: to have more difference from the usual concrete roads.
My observations:
> The textures look too regular. I know that it's hard, because it's a pattern to be repeated, and could look odd; or if it's
too irregular, it can have a concrete feeling. Maybe you can add some bits of different colors (brown, green, tan, i.e.)
> In curves and crossings it has a straight simmetrical shape, making a sort of 90. angle that looks a bit strange. Making it rounder would be better, don't you agree?
They look good overall, but I think that they can still be improved.

I like them; let's hope some patch allows for time-changing roads!!