In every adjustment layer you can paste a greyscale image as a mask. You have to alt-click (or similar, I am on Mac, option-clicking, so not sure) to access the mask, or call it 'jump in'. Also, I am not quite too bright on the theoretical side of things, but some of the layer blending modes do actually what you want when you apply them to a layer without color (not REALLY grey scale, because you would be in a RGB file I guess). Another way to this would be to use a dark version of your file (the not-bright-part), and stack a light version in a layer on top of that. And use the greyscale image as a mask in the top layer. As we say in Holland, there are a lot of roads that lead to Rome. Dunno why Rome, actually.