This is green spill/reflection from the screen, not a white-balance problem. Because the car’s paint is reflective, it is literally reflecting the green background.

What helps during capture:

• Increase distance between the car and the green screen; several feet is recommended, with 8–10 ft often suggested as a minimum.
• Try a polarizer and rotate it for the best reduction in reflections, though it won’t remove spill everywhere on a curved car.
• If possible, shoot a second frame from the exact same camera position with the green covered by a white screen. Use the green-screen shot for masking/replacement, and the white-screen shot for more neutral reflections on the car.

What won’t really fix it:

• Changing white balance. WB affects the whole image and can make everything else shift magenta.
• Lighting alone won’t eliminate reflections from a highly reflective subject.

Typical post solution:

• Despill selectively in post using curves or similar color tools, reducing green only in the affected reflected areas until the paint looks natural.

In practice, the best result is usually a combination of more separation, optional polarizer use, and selective post-production.