Schmitty, drop shadows are something that takes practice and a serious amount of discipline.
The problem is mostly in two areas, once you have good brush control. Most want to see them very well before backing them, since we paint backwards it is difficult to judge how much is enough. A good rule of thumb is once you can see them at all, usually one or two light coats, they are perfect. If they stand out while spraying them, they will be very dark.
For material/color most use window tint paint if available, or transparent black(Auto Air), or very thin black if you use Pactra.
Just remember when painting shadows....On real shadows you can always see the color thru them that they are lying on.
Hope this helps!
...Jim
W.E.D.