Hi
Just to clarify.......
This is the BACKGROUND image.

This the GREEN SCREEN image. The background is to be made transparent

With the background masked out and filled with red. (Could be any uniform colour...Not necessarily green). The Red area in this example will be rendered transparent using the green screen effect. In this example blue couldn't be used as the object is, in itself blue and would also become transparent.

The Cornfield BACKGROUND image is placed on Video Track 1
The GREEN SCREEN Bubble car image is placed on Video Track 2 above the background image/clip
When the sequence is viewed, the overlay clip covers the background.......the cornfield is hidden.

The GREEN SCREEN effect is now applied to the overlay image.clip. The colour to be rendered transparent can be colour picked from the image or in this case, as it is pure Red .....Red can be set with the RGB settings.
As soon as this is done the chosen colour in the overlay becomes transparent..........
Any remaining border can be removed or reduced with a higher Threshhold setting

In this example the Bubble car clip appears against the background cornfield clip......

NOTE:
The BACKGROUND clip/image goes on VIDEO TRACK 1 (Cornfield)
The GREEN SCREENED image/ clip goes on VIDEO TRACK 2 (Bubble car)
Upper tracks cover up lower tracks apart from transparent areas. (Not the other way.)
Hope this makes this clearer.
Nat