Photoshop :: Quickly Selecting Contrast

I have a custom brush defined of my signature that I put on my photos. As I want the signature to be subtle so it doesn't detract from the photo, I use a colour that almost blends with the colours in that corner of the photo. The signature is visible, yet it doesn't attract attention.

As examples, the signature would be a shade of green over grass, blue over ocean, grey over asphalt, etc.

Selecting the color to use is trial-and-error, tweaking it several times so it isn't too bright, too dark, or too contrasting with the background. As each photo needs a different colour, this becomes time consuming after hundreds of images.

Is there a tool or other way in PS (either CS2 or CS4) to select a colour like this more quickly? Perhaps one that picks an "average" colour from the area for the signature, and then tunes it a bit brighter or darker so it will show up?

At the moment I an experimenting with using the Eye Dropper, set to 101x101 sample averaging (the largest available). So with the signature brush selected, I Alt-click to get this average colour. But then I still need to make the colour lighter or darker, otherwise the signature blends in too well, almost invisible. Is their a shortcut key way to lighten or darken the selected foreground colour?