... you can make the brush whatever colour you want, your new layer will be transperant so your brush mark will 'float' over the top of your image...  make the brush as large as you can to begin with, because as you use 'transform' the layer it's stamped on, it will become a little softer (as you interpolate up - aka make it bigger - photoshop has to make up pixels to do this).  I usually set brush opacity to 100%, and then use the layer opacity to get transperancy if needed. 

...I often create an adjustment layer, using curves or levels, to match the shadows to the rest of the image, you can then fill the mask (black) and paint into it (white) where the shadows are to lighten them...  :)

...what I often do, if my max brush size is not big enough, is to stamp the brush on a new layer, then resize it to be larger (using transform, ctrl-t)   :)