The foreground is the subject matter closest to you in your image, that would be the side of the hill in your image. To multi select with any of the tools you mentioned, hold down shift or, you can select the plus sign for the tool to add to the selection, its at the top on the left of the page. Not sure what you are saying here:How do I change a selection to gray instead of the whole image? If you want to select a part of the image, you do just that:]

I never intended to make art.