That is really pretty!
As for grass ... I guess it depends on what you have in mind, but I'd try by drawing paths between the buildings. If you draw the edges of it with small strokes, you get the impression of grass. When you're done, you can colour the areas in between.
Here's an example of what I mean.
Maybe it works better if you put the texture on the grass instead of the roads like I did
Hope this helps. Happy mapping!