This is such an easy technique: take one paper, and fade it into the other. This gives you the chance to introduce some subtle variations into your background, without having to go full out into making something look realistic, and it's great for a soft touch of color if you want it. Here are a couple of examples:
Credits: All Designer Digitals. Papers by Lynn Grieveson: Background from Essential Bases, and blended portion from Catalina (using Anna Aspnes's Fruehling mask). All frames by Katie Pertiet: From the bottom up: Carded Stacked Frames, Filmed Layered Template No. 10 (tape is from this as well), Photobooth Frame No. 2 Sampler, and Little One Layered Template. Fonts: Maszyna Plus and Sidewalk. Journaling reads: "i have seen / gravity / denied / and shadow / fade into / light / a simple act / forbidden / to me"
Oh, what I have to resort to to use these pretty feminine kits! *lol* Everything from Vinnie Pearce's new kit at Pixel Canvas, Shabby Baby.
So, how do you do this? First, you need a mask. A mask is simply a shape, which you will use as a "clipping path"--that is, the image will be mapped directly onto the shape, including it's opacity! It's way cool. =) I used Anna Aspnes's mask in the first layout, but they're really easy to make. Open up a blank canvas, and on a transparent layer, just start painting. You can use a big clunky brush for the middle, but the cooler effect will be given by the edges that fade into nothingness, so make the edges gradually lighter and lighter. I usually start by painting, and then erase a bunch, then paint again... Try using brushes that you've made from scans of some text (it doesn't matter what as it'll be illegible)--those make really nice fine texture.
Once you have your mask all set, place your background paper on a layer beneath it. Then, take a slightly contrasting paper, and place it on a layer *above* the mask. Now, do ctrl+alt+g (in PSE, I think it's just ctrl+g). The top paper will map itself onto the mask, and if you've faded your edges enough, it will gently settle over top the background and give you a nice color spot. Have fun!