You already appear to have tried all the usual methods Alex. In N Gauge I find that a matt painted surface works well enough especially after it has gathered some dust
Do as SFE suggests and try various shades of grey with the odd darker patch thrown in the represent repairs. These can be added with the aid of masking tape to get nice straight edges. Much of the realism is obtained by adding kerbs/pavements in towns and grass verges out in the country.
For the white lines I use one of those fine white marker pens available from most arts and craft shops.