can i suggest building the colours up slowly, roads are rarely just black, they may be built up of greys, and washed out black/ grey colours, dont go for flat colours, they generally look unrealistic
what are the hills made from ?- is it a porous type material, or is it a non porous material ?- try building up the colours slowly, start with the lighter colours first, you can always darken them with multiple coats, its not so easy to make them lighter, ...keep the colours thin, dont use neat paint, build the depth of colour up in several thin coats....this gives the colours some ' weight' and realism
consider using scatter or other applied materials to build into the colour, use say a dark scatter material to represent the tarmac on the road, or use several shades of green or similar to show the vegetation/ surrounding area,
without seeing it it is hard to suggest methods to help , but this is a starter,
what scale are you working to ? - is the location ficticious ? is the diarama for another model to be placed on, or is it a pure topographical model ?
think about the type of terrain you are representing, would the earth run when it rains ?, if so show this in your paintwork ? is there some rockworks visible, show the details and cracks and shadows with multiple layers of colour, building up the colour with the pigments/ paints.
also study somewhere that is similar to the model you are building, see the little details and the way in which the terrain is in real life, then do your best to replicate that, if the road is ful of loose material, ( gravel, shale and similar) then add that to your diarama....
( I am currently helping a pal build a diarama to display his tamiya RC tank, a load of bombed out buildings, rubble, vegetation, oil drums, sand bags, tree stumps, tank track marks in the mud, - you know the stuff....)
it does take time, but if you have invested lots of time and money in the model then why not spend the time and patience in the diarama- it shows your model off to perfection then.....
i am hoping to offer this diarama as a kit on ebay soon with his consent !
any problems or more questions then give me a shout and i will try and help some more,