Here's a tutorial to show you how to lighten a part of your sig.
End result:
Step 1: Begin by opening the sig you want to apply this to. I'm choosing this one.
Step 2: Create a new layer and select your Rectangular Marquee Tool.
Step 3: Create a rectangular box where ever you want on the sig and fill it with white.
Step 4: Change the blending mode to soft light and your done.
Step 5 (Optional): You can change the opacity of the layer if you wish too.
100% Opacity
50% Opacity
Hope you enjoy this tutorial.
More Tutorials: 
