Depends on what you want to do. Do you want more or less opacity? Do you want different colors or what?
Here's an example where I changed the colors of the active (red) and inactive (yellow) frame images at:
Windows & Caption Buttons > Aero > DWM Window > Frames > Window > Top > Active
Windows & Caption Buttons > Aero > DWM Window > Frames > Window > Top > Inactive
Assuming this is a 7 msstyles with transparent frames that you are talking about.