By default, a highlighted window in Exposé is outlined in blue. That outline – and, therefore, the window it’s surrounding – can be hard to spot. But you can change the colour to something more visible. Note that while changing this setting isn’t hard, you do have to edit some system files. So make sure you back up the files in question before you start hacking them.
Start by going to /System/Library/CoreServices. Control-click on the Dock application and select Show Package Contents. In the new window that appears, navigate to Contents/Resources. You’re looking for two files: expose-window-selection-small.png and expose-window-selection-big.png. Make copies of these files to keep as backups.
Now copy both files and paste the copies onto your desktop, with the exact same names as the originals. Open these desktop copies in an image editor (you may need to adjust file permissions), and change the colour. A bright colour like yellow or white is nicely visible; if you’re using Adobe Photoshop, the Hue/Saturation tool can be handy for this. Just be sure to choose the same colour for both files, and don’t alter anything else about the images, such as their physical size.
Once you’re done, save the files to your desktop. (If asked, select no interlacing.) Using the Finder, copy the edited files from your desktop back to the Dock package, replacing the originals. You’ll be asked to authenticate.
Log out and then back in to your account, and activate Exposé to see the changes. If you don’t like the new colour, or if there’s any problem, restore from the backup copies you made; log out and back in again to revert to blue highlighting.