Image Color change to select the color based

It looks for me, like you don't have even the slightest idea about what you are askingfor...

This would only work (and is pretty simple to implement) if color of your car is a solid one - no gradients, no reflections, no shadows - just one pure red, blue or whatever.

Then you could just replace or pixels of specific color on the picture with other color.

If you wish to write code that will automagically find the shape of a car on the picture and correctly modify all shades of red into adequate shades of blue... Well, good luck with that.

Better use Photoshop.