You can change it in the miscdata.txt.
If you don't know what that is: Create a new Folder named "UI" in your Warcraft III folder. Download the file I attached (miscdata.txt) and put it into the UI folder. In the miscdata file, you can edit the color of shadow strike (and many other things). When you save a map with the editor, the map will get the settings from the miscdata.txt.