Hi,
You can use LabelStyle/color. KML colors are of the format aabbggrr
(alpha, blue, green, red). To control label opacity, set alpha to
something less than 'ff', i.e. '7f' is approximately 50% opaque.
Here are some resources on Style, LabelStyle, and color:
http://code.google.com/apis/kml/documentation/kmlreference.html#style
(Style reference)
http://code.google.com/apis/kml/documentation/kmlreference.html#labelstyle
(LabelStyle reference)
http://code.google.com/apis/kml/documentation/kml_tut.html#geometrystyles
(using Styles on placemarks)
Hope that helps, let me know if you have any other questions,
- Roman