Here is a screenshot of the layer blending options in action, from Adobe Photoshop CS (ugh). Click on to see full screenshot.
In this case, I edited the "Blending Options" of the top layer, the trees, and chose to "Blend If" the gray value was greater than 80. As you know, photoshop is both (a) ridiculously expensive to legally own outside of work, ( heavy and takes 5 billion years to load, © does way more than i need, and (d) has the worst history/undo/redo system EVAR! so i don't even use it at work. i love paint.net, this is just one little feature i miss.
Hopefully this illustrates what I had in mind!