Hi Fran,

Here's an image showing the eyedropper and gradient tools. Use the eyedropper tool as Claymor suggests and select the gray level from your selection (road). The gray level you selected will become the foreground color and you can then paint, or fill the area inside your selection. You don't need to use the gradient tool, I just mentioned it in case you wanted some slope in the road.

Hope this doesn't confuse you more.