This is a very complicated situation. First, we would need to know the color the car is currently along with the color you would like the car to be. The reason being is that it's much easier to do a hue shift from, say, red to orange or blue to purple than it is to change color straight out from black to red for instance.
Let's say that you want to change the color of the car from yellow to green but a shade of green that is relatively the same luminosity but not in the same hue family, like electric green.
1. Create a new solid layer with the color you want the car to be more or less.
2. Mask this new layer to fit the outline of the car.
3. Set the blend mode to color. You'll notice that this probably get you 75% of the way there.
4. Copy your original video layer above your current two layers, add the mask and change blending mode to saturation. This will help pull the reigns in on the color and desaturate the shadows for a more true-to-life look.
5. Create an adjustment layer and copy the mask from the solid layer to the adjustment layer. Add an HLS and curves adjustment. Chances are it'll need to be tweaked further.
This is probably one of the easier ways of doing a color change that looks good.
I agree that having the footage will get you more input.
Of the top of my head, without seeing it, I would say that if you have a clean plate (or if you can create one), you can pull a difference key to get a mask for the people getting in the car and use PSD to create a plate where the car has the color you want. Cut the car out of that, composite behind the people getting in the car, and match grain.
An alternate solution (as there are many) would be to apply Hue/Saturation to your layer. Change the channel from master to one of the channels that most closely represents your car's color. Swing the hue to make the car more the color you want. Tweak the paremeters of the color channel to get it narrowed down to your car's color.
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