So - you have two separate scenes. And, if I understand well, CarColor is the node inside Car. So - you have to reach Car node and then get from it CarColor node.
If they're loaded and append to the main world somewhere in the structure, you may do in such way:
var car_color_node = get_world().get_root().get_node("Car").get_node("CarColor")
car_color_node.modulate = color
And you may also do in connected way:
var car_color_node = get_world().get_root().get_node("Car/CarColor")
car_color_node.modulate = color
You may also create variable in your color picker and set it to the node, on which you are working. There're many options.