Cobbled

So my second attempt at creating cobbles with a biro and some clay seems to have produced a fairly convincing pattern. Of course a cobbled surface would never be white, so the next step is to paint them. Now the painting is also an experiment, and I'm not entirely sure I've figured it out yet. Here you can see my first attempt.

The process here was to spray everything with matt black from a aerosol can. Once that had dried I dry brushed the cobbles with three Model Color paints; London Grey, Ivory, and Tan Earth. I then added a small amount of black wash to the cobbles between the rails to simulate oil etc. dropped from the locomotives. Now while I quite like the effect I have a feeling the whole thing is a bit dark, and probably too dark to be believable. I don't want to go for a really light coloured surface but I think it still needs some work. Any thoughts or suggestions?