looks wierd because car is transparent, try it with masking.
btw. i made this with chrome key, but i forgot how can someone help
edito coloro testo
If its done with Chroma Keyer, you prolly created two layers with the Chroma keyed layer on top. Then you used Color Balance for instance to change the colors of the layer below.
I prefer Color COrrector Secodary tho because it saves you an extra layer. I would only use this effect on cars with a very distinctive color, like the one of a cabbie or a green infernus since they are easy to edit