From the Fiddler Book:
Each of these Preferences controls a color used by Fiddler in the UI. Set the preference to either a hexadecimal color string (e.g. #FF0000 equals Red) or a .NET color string. :
fiddler.ui.Colors.LoadedFromSAZ
fiddler.ui.Colors.AutoResponded
fiddler.ui.Colors.QuickExec
fiddler.ui.Colors.QuickExecText
fiddler.ui.Colors.ColumnDuplicate
fiddler.ui.Colors.ImageBloat
fiddler.ui.Colors.ImageBloatMortar
The color you're looking for is LoadedFromSAZ, which defaults to WhiteSmoke.
You can adjust prefs by typing about:config in QuickExec and using the editor. Most of these require restart for the color change to take effect.