[oldeguy]

Ideally, painting your spoiler would be the best.
Putting a black stripe on the car would look good, if, the body side mouldings are changed to yellow and the black trim on the back of the car is changed to yellow as well.
If that can't be done, either you should wait to paint the entire vehicle, or somehow, if it looks good, tie in the black spoiler to the black body side moulding. I think, a stripe from the spoiler can look good, if the proper amount of thought is put into it.