Open your image of your car, im going to use my focus st vmod that ive prevously done.
Once its opened duplicate the layer “background” and rename your duplicated layer to new color.
Now with the “new color” layer selected, select the pen tool and start drawing round the car, making selections to the bits you want to change the color of, also make sure you dont select other things like the grills, wheels, windows etc… and take your time, it is a slow process but you will get there. (im not going to explain how to use the pen tool, but if you got “HERE” there is a good tutorial to explain it. )
Once you’ve gone round the whole car you should have something like this.
Now for the color, keep it selected and goto the toolbar at the top and click “image > adjustments > hue/saturation” or alternatively “ctrl + u”
Now with the new dialog open put a “tick” in the “colorize” box. now drag the “hue” slider all the way to the left or type in “0”, now slide the “saturation” slider in the middle or type in a number, like “60” your car should now be a red color.
Move both sliders to obtain different colors, you can basically make any color from the sliders. when your happy with your color click ok.
Now press “ctrl + D” to deselect, you might need to touch up in places where the old color shows throu, also if theres things like reflections in floors and metals dont forget to change the reflection color.