You might want to check out a craft store such as AC moore. They have different "paint" kits to do faux finishes. I don't know if they are plastic friendly, but I have used them on wood and they do produce a nice look.
I don't know the look you want as marble can be either shiny or flat on building exteriors. If a flat look is desired there are some very nice granite paints available in spraycans. I haven't used them on styrene but they have proved quite compatible with other plastics such as pvc and acrylics.
They have a texture that includes small color variations within and are available in several colors. The road below was sprayed with black granite paint and then oversprayed lightly with some light weathering to leaven out the color variation a bit and more closely resemble pavement. I know that there are also green, grey and tan colors available, perhaps others.
The white line was painted on afterwards by masking off the rest of the road. Might work for you if granite is okay.