This is a cool little tip for people on Snow Leopard. In the new update you will notice that when you use Exposé there is a small blue glow around any windows which you select. This is shown in the image below. It is very simple to change the colour of this image. All you need is to find the correct file and then change the Hue or Saturation in your favourite image editor. Since this tip involves accessing system files you will need to be careful not to damage anything which is needed by your system.
The first step is to locate the files which are used by your system. These are located in System > Library > CoreServices in the list, right click on “Dock” and select “Show Package Contents”, then select Contents > Resources. In this list of files there should be two files called “expose-window-selection-big.png” and “expose-window-selection-small.png”. These are the two files associated with the blue glow. Backup these files by either copying them to a different location or duplicating the files in the folder and giving them a different name. You will probably have to enter your admin password to do any changes.
The net step is to change the colour. Open up the images in your favourite image editor (you may need to copy them out first), you can use Photoshop, Pixelmator or any other image editing app. Try and use one which allows transparent images so you can keep the glow. Make sure you keep the same dimensions and file types so you don’t end up breaking the Dock app.
When you have finished copy the new images back into the Resources folder in the Dock folder which you opened previously. You will need to restart for the changes to take effect. You will now have a different coloured Dock glow.