As Liz instructed, you use the magic wand to select only parts of the paper. Make sure "contiguous" is unchecked, so it will select that particular color all over the paper layer. Then use the delete key to delete those selected spots. Repeat if necessary with another selection.
Or, you could also use the eraser, with a grungy, open brush and lower the flow and/or opacity to get the same sort of effect. That's what I did on my layout.
Hope this answers your questions.