Welcome @alp !
The one example that comes to my mind:
With this you can use the picked color and apply it to the sprite (GUI image node’s color or sprite’s tint) depending on if you want to make the car in GUI or as a sprite in game object 
If you will tinker a little bit with it you should be able to create a simple example, where you pick a color and the picked color is applied to sprite/gui node. If you will face any problems, just let us know 
Edit: I tested the color picker by @ross.grams, but it uses custom render script and material to modify displayed colors, so you will probably need to digest into this topics a little bit.
If you are looking for an even simpler solution you can think about for example - predefined pallete presented to user to choose a color from (it could be as simple as showing few gui nodes and applying tint to sprite/ color to another gui node with car parts.) or showing a pallete image and picking position where user clicked and calculating the picked color basing on the position. There are few possibilites, so please, consider, what could be the best for you
If you are just starting, make a simple prototype, where you choose between e.g. 2-3 predefined colors and according to the selection tint the sprite